Sequence of events in biofilm formation on surfaces in seawater.
Four/1997 1

Naval Research Reviews publishes articles about research conducted by the laboratories and contractors of the

Office of Naval Research and describes important naval experimental activities. Manuscripts submitted for

publication, correspondence concerning prospective articles, and changes of address, should be directed to Code

00PA, Office Naval Research, Arlington, VA 22217-5660. Requests for subscriptions should be directed to the

Superintendent of Documents, U.S. Government Printing Office, Washington, DC 20403. Naval Research

Reviews is published from appropriated funds by authority of the Office of Naval Research in accordance with

Navy Publications and Printing Regulations. NAVSOP-35.

CHIEF OF NAVAL RESEARCH

RADM Paul G. Gaffney, II, USN

DEPUTY CHIEF OF NAVAL RESEARCH

TECHNICAL DIRECTOR

Dr. Fred Saalfeld

CHIEF WRITER/EDITOR

William J. Lescure

SCIENTIFIC EDITOR

Harold Guard

Kenneth J. Wynne

MANAGING EDITOR

Norma Gerbozy

ART DIRECTION

Cynthia Nishikawa

Jorge Scientific Corporation

Office of Naval Research

Four/1997

Vol XLIX

18

Profiles in Science

About the cover

A panel immersed in Pearl Harbor for seven weeks shows nearly 100% cover by tubeworms

(Hydroides elegans). Tubeworms are a problem biofouling organism in tropical and subtropical

waters. Photography courtesy of Dr. Michael G. Hadfield, Celia M. Smith, Catherine R. C. Unabia,

University of Hawaii.

Articles

Departments

2

Introduction

K. J. Wynne and H. Guard,

Guest Editors

4

Toward Minimally Adhesive

Surfaces Utilizing Siloxanes

A. E. Mera, R. B. Fox, S. Bullock,

G. W. Swain, M. P. Schultz, P. Gatenholm,

and K. J. Wynne

9

Biofouling, Fleet

Maintenance and

Operational Needs

G. S. Bohlander and J. A. Montemarano

19

New Technologies for

Surface Analysis to Assess

Biofouling Resistance

J. A. Gardella, Jr., K. J. Wynne, T. J.

McCarthy, J. F. Rabolt, and G. Belfort

35

Exterior Hull Coatings in

Transition: Antifouling Paints

and Fouling Release

Coatings

J. R. Griffith and J. D. Bultman

39

Silicon Fouling Release

Coatings

H. L. Vincent and G. G. Bausch

46

Field Evaluations of NonToxic Antifouling Coatings:

New Field Technologies and

Performance Criteria

G. W. Swain

51

Selection, Design and

Delivery of Environmentally

Benign Antifouling Agents

D. C. Sundberg, N. Vasishtha, R. C.

Zimmerman, and C. M. Smith

60

Certification of Properties of

Nontoxic Fouling-Release

Coatings Exposed to

Abrasion and Long-Term

Immersion

R. E. Baier, A. E. Meyer, and R. L.

Forsberg
2 Naval Research Reviews

Introduction

Dr. Kenneth Wynne, Guest Editor, Physical Sciences

S&T Division, Office of Naval Research

Dr. Harold Guard, Guest Editor, Biological and Biomedical S&T Division, Office of Naval Research

In this issue of Naval Research Reviews, there are eight

articles that show the direction the Office of Naval Research

(ONR) has taken to develop new and generic means of intervention to inhibit or minimize biofouling on Navy ships.

Biofouling, the growth of barnacles, seaweeds,

tubeworms and other marine organisms on the hulls of oceangoing vessels, costs the international marine community billions of dollars a year. Most of this money goes for the extra

fuel needed to overcome the increased drag on vessels. Some

of it is spent for hull cleaning and repainting and for the

upkeep on propulsion equipment. A tiny amount is invested

in the search for better biofouling inhibitors.

Under certain conditions a ship with a mature biofilm

(a layer of slime that forms on a wet surface due to microbial

activities) of 24 months growth can require 8 percent more

shaft horsepower to move as fast as a comparable ship with

no biofilm. Between 18 and 22 percent of the propulsive

fuel used by the Navy goes to overcome the drag caused by

biofilms and organisms such as tubeworms and barnacles.

In addition to the huge financial burden, biofouling

has an environmental cost as well. The use of copper-based

coatings to combat biofouling over the past century and

organotin-based coatings introduced in recent decades have

resulted in the release of significant quantities of both metals into the marine environment. Even relatively low levels

of copper and very low levels of organotin compounds have

been shown to inhibit the development of fish and invertebrates. Coatings which release tributyltin oxide (TBT) are

restricted in 14 states and many European countries.

The Navy operates in many seas and ports and the

biofouling problems are different in each location. For example, barnacles are a major problem along the East Coast,

and tubeworms cause trouble in Hawaii. Today, the Navy

has domestic test sites in Hawaii, North Carolina and Florida

to study antifouling strategies. Test sites were also established in the following foreign locations: Hong Kong,

Singapore, India and the Mediterrranean.

In the early 1950’s, the Office of Naval Research began funding basic research programs which were aimed at

understanding the life cycle and biology of wood borers and

hard-foulers like barnacles and mussels. It was hoped that

such understanding would permit interference with the development of these organisms. Under applied research

programs, the organisms were deterred through use of paints

such as the TBT and copper-based coatings. These paints

have been successful but function through toxicity.

Today, ONR is funding research to attack the “gluing”

techniques these many organisms use to attach themselves

to the hulls of ships. All the fouling creatures have one common characteristic — they adhere. Research is underway to

better understand the principles that govern bioadhesion. At

ONR, Dr. Harold Guard and Dr. Kenneth Wynne oversee a

program to develop non-polluting antifouling coatings with

an improved operational lifetime. Also, they are establishing reliable field test and evaluation systems that will hasten

the transition of the new coatings to the fleet. The new studies are focused on bioadhesion, which involves exopolymers

and surface polymeric properties. The focus is on the chemistries at the interface between the biofoulers and the surface. The biofoulers contain polysaccharides and

glycoproteins which interact with the surface chemistries to

develop the bioadhesion. In other words, the degree at

bioadhesion is related to polymer surface structure morphology and energies present, and by exopolymer structure and

chemistry.

Ann E. Mera, et. al. in the first article compare the

surface and fouling behavior of representative coatings from

two well known polydimethylsiloxane (PDMS) classes,

alkoxysilane cured networks and Pt cured networks. The

representative alkoxysilane-cured coating is RTV11®, the

“topcoat” of the Naval Research Laboratory duplex coating.

The curing chemistry is conventional RTV (Room Temperature Vulcanizing) PDMS network formation, achieved by

condensing silanol-terminated PDMS chains with a multiethoxysilane crosslinker, in this case polydiethoxysiloxane

with 40% SiO2 content (ethylsilcate 40). Variables examined include PDMS molecular weight, tri- versus tetra- functional crosslinker, and reactant Si-H/vinyl ratio.

The second article by G. S. Bohlander and J. A.

Montemarano describes a new generation of fouling control

coatings that are free of heavy metals. These materials function by discouraging fouling settling, and producing a poor

surface for fouling adhesion. Automated underwater vehicles

offer promise providing in-water hull cleaning and maintenance monitoring for Navy ships to reduce maintenance costs.

Future needs for hull maintenance include non-toxic fouling

control coatings that can be applied in the industrial environment, will self-clean with low speed ship movement, and

will provide 5-7 year service. These coatings should be renewable for service out to 15 years, and then be easily removable.

In the third paper, Gardella et. al. provide a review of

the advances in instrumentation, measurement science and

methodology which are vital to the analysis of polymer surfaces. There is also a short description of advances in methodology which are driven by problems in polymer materials

surface chemistry. The use of these methods is critical in the

development of model systems and standard materials. A

selection of work sponsored by the ONR-Molecular Interactions at Marine Interfaces program is also mentioned. Recent ONR research in the application of these methods

resulted in the development of model systems and actual candidate coating materials.
Four/1997 3

Griffith and Bultman in the fourth article outline the

efforts at the Naval Research Laboratory, Washington, D.C.,

since 1970 to develop “fouling release coatings.” Since the

introduction of TeflonTM, to the general public as an antistick

coating for cooking utensils, inventors have dreamed of coating the exterior hulls of ships with substances so that barnacles would fall right off. Such antiadhesive coatings, in

principle, have a number of very attractive features such as

absolutely no environmental degradation and long, effective

life. However, efforts to implement this concept in a practical manner are fraught with many difficulties.

The fifth article by Vincent and Bausch describes research in the development of silicon fouling release coatings. Since the Antifouling Paint Control Act of 1988

effectively prohibited the use of the tributyltin self-polishing copolymers for Navy use, there has been a change to less

toxic copper oxide ablative coatings. However, such coatings do not provide the level of performance attained by the

organotin self-polishing coatings. Research continues with

fouling control coatings which are based on the PDMS backbone and offer a non-toxic approach to biofouling control.

They rely upon the physical nature of the coating to retard

the onset of fouling and provide easy release when fouling

does occur. Additional development needs to be completed

to identify commercially viable means of combining the antifouling agents and PDMS based coatings. Also, testing

should continue on ideas for improving the overall toughness and fouling release properties of the PDMS coatings.

The sixth paper by Swain addresses the use of the static

immersion panel evaluation methods for providing new insights into the performance of non-toxic coatings. It is now

possible for researchers to quantify how modifications to their

formulations alter the strength of biofouling adhesion, and

for the Navy to select the coatings with the best foul release

properties. Research is currently underway to relate

biofouling adhesion strength to mechanical cleaning. This

knowledge should help in the development of non-toxic antifouling coatings and ship hull maintenance practice that

will remove the necessity for the use of biocides.

Sundberg et. al. in the seventh article describe the important ONR goal of understanding and exploiting natural

agents and their analogs that promise effective biofouling

control without polluting the environment. Nature has

evolved a variety of strategies that prevent or minimize fouling of natural surfaces in aquatic environments without resulting in permanent environmental degradation or

accumulation of refractory and toxic substances. Some organisms use physical means, including the sloughing of the

outer tissue layer, the generation of an external surface that

minimizes bioadhesion, and the production of secondary

metabolites that can deter potential fouling organisms and

predators. The chemical species employed as defensive

agents are as diverse as the organisms manufacturing them.

Those known agents include elemental vanadium inorganic

acids, and a variety of organic compounds including saponins,

terponoids and phenolic acids.

Lastly, the eighth article by Baier and Farsberg describes brush abrasion tests to sequentially evaluate foulingrelease coatings after increasing numbers of wet brushing

cycles. Standard tests for abrasion and wear developed for

paints are not applicable to elastomeric coatings. Coatings

which contain elutable oils have displayed further resistance

to initial colonization by barnacles and zebra mussels, but

this early benefit does not necessarily carry over to the brushremoval forces required for cleaning of the once-fouled coatings.

This research combines the efforts of Dr. Harold Guard

and Dr. Kenneth Wynne, Division Directors at ONR. The

cost incurred by the Navy to combat biofouling will escalate

significantly with new environmental and health laws governing the use, application, and removal of environmental

toxins like those present in materials used by the Navy today to coat ship hulls and submerged platforms. This ONR

program will develop new and generic means of intervention to inhibit or minimize biofouling on Navy ships.
4 Naval Research Reviews

Introduction

This paper is aimed at the correlation of physical surface characterization data on silicone “fouling release” polymer coatings with measurements of the tenacity of adhesion

of hard fouling in the marine environment. Nontoxic coatings with minimally adhesive surfaces with respect to

biofouling are of interest to Navy, from both environmental

and economic standpoints.1 The accumulation of biofouling

(algae, silt, grass, barnacles, worms, etc.) on a ship’s hull

increases weight and drag, leading to higher fuel consumption and operating costs. Bridging of fouling on heat exchanger face plates blocks inlet water and causes inefficient

cooling of power units. Currently used hull antifouling coatings contain species such as tributyltin oxide or copper oxide and function via leaching of the toxicant into the marine

environment. The resulting environmental hazards of introducing such toxicants into the marine ecosystem include disruption of natural ecocycles for many commercially

important shellfish and pollution of entire food chains.2

Removal and disposal of toxicant-containing coatings from

ships also pose separate environmental hazards, driving up

the cost of refurbishment.

In order for a polymeric coating to function successfully as a fouling release coating, the surface must: (1) have

a low surface energy to minimize chemical interactions; (2)

have a low glass transition temperature, Tg, to minimize

mechanical locking of a prospective fouling organism; and

(3) be temporally and chemically stable in water; that is,

conditions (1) and (2) must not change with immersion time

in water. Poly(dimethylsiloxane) (PDMS)-based coatings

have properties which meet requirement (2) and satisfy requirement (1) as well as any practical polymer developed

thus far. However, there is an important challenge is in meeting requirement (3) and this is addressed in this paper.

This paper compares the surface and fouling behavior

of representative coatings from two well known PDMS

classes, namely alkoxysilane cured networks and Pt cured

Toward Minimally

Adhesive Surfaces

Utilizing Siloxanes

Ann E. Mera, Robert B. Fox, and Steve Bullock, Materials Chemistry Branch, Naval Research Laboratory, Washington, DC

Geoffrey W. Swain, Division of Marine and Environmental Systems, Florida Institute of Technology, Melbourne, FL

Michael P. Schultz, Division of Marine and Environmental Systems, Florida Institute of Technology, Melbourne, FL

Paul Gatenholm, Department of Polymer Technology, Chalmers University of Technology, Göteborg, Sweden

Kenneth J. Wynne, Physical Sciences S&T Division, Office of Naval Research, Arlington, VA
Four/1997 5

networks. The representative alkoxysilane-cured coating is

RTV11®, the “topcoat” of the NRL the NRL duplex coating.3

 The curing chemistry is conventional RTV (Room Temperature Vulcanizing) poly(dimethylsiloxane) (PDMS)

network formation, achieved by condensing silanol-terminated PDMS chains with a multi-ethoxysilane crosslinker,

in this case polydiethoxysiloxane with 40% SiO2 content

(ethylsilicate 40). The second class of coatings is based on

the well known Pt catalyzed hydrosilation of divinyl terminated PDMS chains with multifunctional silane crosslinkers.

Variables examined include PDMS molecular weight, triversus tetra- functional crosslinker, and reactant Si-H/vinyl

ratio.

One effective way to assess the temporal stability of

coatings surfaces in water is dynamic contact angle (DCA)

measurements.4 We have used this method to determine the

underwater surface dynamics of other coatings,

poly(dimethylsiloxane-urethane-urea) segmented block copolymers5

 and poly(methyl methacrylate)-g-poly(siloxane)

copolymers.6 Other methods used to determine the chemical stability of coatings over time in water include weight

loss, atomic force microscopy (AFM), and surface analysis

by electron spectroscopy for chemical analysis (ESCA).

Combined, these methods give insight into the mechanisms

by which silicone-based surfaces change when immersed in

water. Below, our surface analytical results for our representative coatings are correlated with field tests for fouling release behavior.

Experimental

Materials

GE RTV11® was obtained from Read Plastics. The

Material Safety Data Sheet indicates the composition as follows: hydroxy-terminated PDMS (60- 80%)(Mw 26,000, Mw/

Mn = 1.7, by GPC vs. polystyrene standard), CaCO3 filler

(10-30%), Ethyl Silicate 40 (1.6%). Vinyl-terminated PDMS

(200 cs and 1000 cs), methyltris(dimethylsiloxy)silane

(trifunctional crosslinker), tetrakis(dimethylsiloxy)silane

(tetrafunctional crosslinker), and platinum

divinyltetramethyl-disiloxane complex (catalyst) were obtained from United Chemical Technologies, Inc.

RTV 11® Samples

The material was mechanically stirred for five minutes prior to the addition of 0.1%, 0.25%, and 0.5 wt%

dibutyltin dilaurate (DBTDL) catalyst. The mixture was

stirred for an additional 10 minutes then degassed in a vacuum

oven at room temperature for 10 minutes. Samples were

obtained by dip coating 22 x 50 x 0.15 mm glass cover slides.

Curing was accomplished in either high humidity (90% R.H.)

or ambient humidity (ca. 20% R.H.). Minimal differences

in wettability were observed for samples cured at high or

ambient humidity utilizing 0.5 and 0.25 wt% DBTDL.

Samples cured with 0.1 wt% DBTDL were cured only under

high humidity conditions.

PDMS Networks Via

Hydrosilation

The networks were prepared by mixing appropriate

amounts of degassed vinyl-terminated PDMS and tri- or tetrafunctional crosslinker with an equal volume of hexane, followed by the addition of 2-10 ppm catalyst. The reaction

mixture was stirred by hand for four minutes. Samples were

obtained by dip coating 22 x 50 x 0.15 mm glass cover slides.

The coatings were allowed to cure 48 hr prior to measurement.

Surface Characterization

DCA measurements were performed on the polymer

surfaces using a Cahn DCA Model 312 analyzer. Water for

the dynamic contact angle measurements was purified to Type

I with a Barnstead Nanopure Bioresearch grade system and

had an average resistivity of 18 M3-cm. The surface tension

of the contact angle probe water was checked daily and had

a typical value of 72.6 dynes/cm. The rates of immersion

and withdrawal were 100 m/s; top dwell times were three

minutes and bottom dwell times ranged from zero to three

minutes. Each advancing/receding contact angle data point

is the average of the last three immersion/withdrawal cycles.

For long term immersion testing, samples were stored in either nanopure water alone or nanopure water spiked with

200 ppm sodium azide (as a biocide7

); solutions were replaced weekly. AFM was performed using a Nanoscope IIIa

Atomic Force Microscope in tapping mode using 125 mm

silicon cantilevers with typical resonance frequencies, Fo, of

271-325 Khz. The field size was 30 X 30 m. Phase contrast

for AFM was performed on a Dimension 3000 large sample

AFM. ESCA was performed using a Perkin Elmer PHI 5500

equipped with a Mg K X-ray source at take-off angles of 20

and 70 degrees from the plane of the sample and with 4-5 eV

flood gun charge compensation. Binding energy shifts were

determined by referencing the highest peak in the C1s spectrum to either 292.0 eV for CF2

 groups or 285.0 eV for hydrocarbon.

Fouling Adhesion

Field tests were performed at NRL, Chesapeake Beach,

MD and at The Florida Institute of Technology, Melbourne,

FL. Coated panels were prepared similarly to DCA slides

except that only one side of each fiberglass panel was coated.

Immersion tests are ongoing, results reported are those obtained after monthly withdrawal, inspection, and cleaning of

the panels.
6 Naval Research Reviews

Results and Discussion

RTV 11®Surface Composition

Under the water sparse conditions of cure, hydrolysis

and particularly condensation reactions which effect cure of

RTV11® are relatively slow. 7 For the analytical studies described below, samples were allowed to cure a minimum of

twenty-eight days, to ensure full cure throughout the entire

film. X-ray spectroscopy for chemical analysis (ESCA) revealed Si, C, and O in the expected ratios, but no Ca could

be detected. Figure 1 shows schematically the surface composition of RTV11® based on these and other analytical results.

Stability of RTV11® in Water

Long-term water immersion studies were performed

on fully cured samples with varying catalyst concentrations.

The first indication of chemical instability of RTV11® coatings came from mass loss studies (Figure 2). Error bars are

included for the 0.1 wt% catalyst samples only; similar error

ranges are observed for the other samples. The total weight

loss over 70-85 days is 0.1-0.3%. Differences in weight loss

for different catalyst concentrations are significant only after more than two months immersion. Samples with 0.25

wt% catalyst show the lowest mass loss after this time.

To elucidate the nature of the mass loss process, tapping mode atomic force microscopy (TM-AFM) imaging was

performed on samples cured with 0.25 wt% catalyst. The

pristine film has a roughness coefficient of 45 nm after three

months immersion in water. The roughness coefficient increased to 95 nm, with evidence of a non-uniform erosion

process. Scattered pits of various size and depth appeared

on the surface, indicating mass loss occurs in highly localized areas of the surface. We are currently attempting to

calculate the void area from AFM and correlate this with

mass loss, to determine if all of the mass is lost through pitting or if there is additional overall film erosion.

The results from TM-AFM explain why little change

in the ESCA spectrum was observed in water aged samples.

A relatively large area is interrogated by ESCA (ca. 4 mm2

)

so the pits introduced in water-aging did not perturb a large

enough area fraction to effect ESCA results. Light microscopy was not effective in identifying the presence of pits as

the white pigmentation of CaCO3 makes the films highly

reflective. Wettability data are also not sensitive to the long

term pitting-erosion process, though a dramatic short time

(24 hr) increase in wettability is observed for RTV11®

samples which we ascribe to diffusion of polar, near surface

PDMS chain ends (terminated with Si-OH and Si-OSin

(OH)m ) to the surface.

PDMS Networks Via

Hydrosilation

Two different divinyl-terminated PDMS samples (molecular weight data: 200 cs = 9400, 1000 cs = 28,000) and

two different silane crosslinkers (trifunctional = XH3;

tetrafunctional = XH4) were used to form networks with a

variety of crosslink densities. Surface analysis by ESCA

showed the expected ratio of Si, C, and O peaks for PDMS.

The results of wettability for these PDMS networks are given

in Table 1, arranged in order of highest to lowest crosslink

density. Within experimental error, no measurable differences in advancing and receding contact angles are observed

with varying PDMS molecular weight or crosslinker.

In each data set shown in Table 1, the Si-H/vinyl ratios were varied, from 1.1 to >2.0. Ideally, a Si-H/vinyl ratio

of 1 should result in a fully cured network. In practice, this

is not the case; Si-H/vinyl ratios >1 are needed to obtain

non-tacky fully cured networks. Reactions other than

hydrosilation occur which consume excess Si-H

functionalities,8

 including reactions with water and/or oxygen to from Si-OH species which can further condense to

form Si-O-Si bonds.9 For all of our data sets there was no

systematic trend of either the advancing or the receding contact angles with increasing Si-H/vinyl ratio. Therefore, the

excess Si-H functionalities must react in such a way as to

Figure 1

Schematic of Near Surface Composition of RTV11®.

Figure 2

RTV 11® immersion mass loss data for films prepared with

0.1, 0.25, and 0.50 wt% catalyst.
Four/1997 7

not contribute any hydrophilic nature to the surface. Formation of Si-O-Si bonds, as outlined above, is one possible route.

We are currently investigating which side reactions are occurring, either during or after network formation, to explain

the apparent lack of surface hydrophilicity with increasing

Si-H/vinyl ratios.

Within experimental error, there is no drop in receding contact angle or measurable weight loss with time immersed in water (up to 2 months). Thus, in contrast to

RTV11®, Pt- cured divinyl coatings exhibit remarkable stability in water. Figure 3 illustrates the stability of a representative sample of these networks, showing no systematic

decrease in receding contact angle for 1000 cs PDMS + XH3,

over 28 days immersed in water. The Si-H/vinyl ratio was

also varied in this example, from 1.18 to 3.07, and it can be

seen there is no measurable trend along this axis. The stability of these coatings to acid catalyzed hydrolysis is also exceptional. We observed no changes in either the advancing

or the receding contact angles over 12 hours immersion in

0.1N HCl for networks synthesized using 1000 cs PDMS

with both tri- and tetra- functional crosslinkers.

Field Tests of Strength of

Adhesion of Hard Fouling

Field tests have been initiated at Melbourne FL on panels coated with our PDMS networks synthesized via

hydrosilation and RTV11® for comparison of fouling and

fouling release behavior. Barnacle adhesion is estimated by

the method of Swain.10 This mechanical test gives a quantitative measure of barnacle adhesion to coatings. In Table 2,

preliminary data is presented for 1000 cs PDMS + XH3, along

with data for RTV11® and FEP Teflon®, the best non-silicone coating for fouling release. The data show that Pt-cured

divinyl silane (1000 cs PDMS + XH3) is superior to RTV11®

for ease of barnacle removal.

Conclusions

This study has shown that the surface of RTV11®, an

alkoxysilane cured PDMS material, unstable when immersed

in water for three months. Mass is lost, and the contact angle

hysteresis undergoes substantial changes with time of immersion. TM-AFM reveals a nonuniform micron scale erosion or “pitting” phenomenon occurring at the surface. In

contrast, surfaces of PDMS networks synthesized via Pt

catalyzed hydrosilation exhibit remarkable stability when

immersed in water No decrease in wettability and no measurable weight loss occurs for up to two months. Within the

range studied, the surface properties of these Pt-cured networks are relatively insensitive to initial Si-H/vinyl ratio and

to crosslink density.

The instability of RTV11® is perhaps not surprising

considering the presence of CaCO3 filler and the crosslinking

chemistry. For RTV11®, the network is formed via condensation of Si- OH from terminal PDMS functionality and SiOH from the hydrolyzed alkoxysilane. The PDMS chain is

stable to water at ambient temperature, but the siliceous phase

produced by the alkoxysilane may be susceptible to hydrolysis. While CaCO3

 filler is not detected by ESCA in the

RTV11® surface, it may be present below detection limits

which are estimated to be 1- 0.5 wt%. Any near surface

CaCO3 would provide a site for attack by water, as CaCO3 is

soluble in water. Whatever the cause of RTV11® microroughening and erosion in water, this process likely facilitates the adhesion of marine organisms.

In contrast, the surface of Pt-cured PDMS is chemically stable in water. This follows from the cure chemistry

in which non-hydrolyzable C-Si and C-H bonds are formed.

We believe that the better fouling release of Pt-cured PDMS

Figure 3

1000 cs PDMS + XH3 ; wettability as a function of Si-H/vinyl

ratio.

Table 1

Average Water Contact Angles for PDMS Networks Via

Hydrosilation

Table 2

Barnacle Adhesion Data
8 Naval Research Reviews

is due to the chemical stability of a surface which is principally PDMS. By its chemical nature, Pt-cured PDMS is probably the ultimate PDMS coating for fouling release and a

standard by which other PDMS systems may be evaluated.

It must be kept in mind that even with barnacle removal requiring only 5.4 Å 3 psi from Pt-cured PDMS, the surface of

these panels readily foul but are easily cleaned.

At the present time there are a number of features of

Pt-cured PDMS which make it impractical for shipyard application. Materials approaches to improve Pt-cured silicone based coatings include more robust catalysts for

hydrosilation and the investigation of novel reinforcement

methods to improve toughness, which is a weakness of most

silicone coatings.

Materials approaches to improve alkoxysilane cured

PDMS include modification of the crosslinking chemistry

and improved reinforcement materials for improved stability in water. Improvements in alkoxysilane cured PDMS systems will be important because this coating chemistry is

already in limited application as fouling release coatings.11

Acknowledgments

This research was supported by the Strategic Environmental Research and Development Program (SERDP) and

the Office of Naval Research Scientific Officer Research

Program (SORP). The authors thank Dr. Tai Ho for GPC

data.

Biographies

Dr. Ann E. Mera earned her Ph.D. in Polymer Science

from The Pennsylvania State University in 1986. She is a

research chemist in the Materials Chemistry Branch at the

Naval Research Laboratory (NRL), Washington, D.C. Dr.

Mera’s research interests include fluoropolymers, siliconecontaining polymers, and low surface energy coatings for

naval applications.

Dr. Robert B. Fox has over 50 years experience with

the Chemistry Division at NRL, where he worked in polymer synthesis, photophysics, and sonic properties. His current interest is in hydrolytically stable silicone networks. Dr.

Fox has served on the Board of Directors of the American

Chemical Society and was a founding member of the Macromolecular Nomenclature Commission of the International

Union of Pure and Applied Chemistry.

Steven Bullock is a master’s candidate at George Mason University in Chemistry. Currently, he is working at the

Naval Research Laboratory through a grant from the Office

of Naval Research funded by SERDP. His research interests

lie in the area of low surface energy polymers. Mr. Bullock

has a B.S. from Virginia Polytechnic Institute and State University and will complete his master’s in August, 1998.

Dr. Geoffrey Swain, see page 50.

Michael P. Schultz is currently a Research Engineer in

the Division of Marine and Environmental Systems at Florida

Institute of Technology (FIT). He received a B.S. and M.S.

Degree in Ocean Engineering from FIT and is currently completing his Ph.D. (finish Spring 1998). His research interests include biofouling test method development, turbulent

boundary layer flows, hydrodynamic force measurement on

fouling organisms, and marine corrosion.

Dr. Paul Gatenholm is Professor of Polymer technology at Chalmers University of Technology in Gothenburg,

Sweden and Adjunct Professor of Biomaterial engineering

at Virginia Tech. His research interest is surface engineering of polymers and fibers and special interest is in an interface between synthetic polymers and biological environment.

He has published more than 80 scientific papers and 60 conference proceedings and has several patents. He has recently

been visiting scientist at NRL.

Dr. Kenneth J. Wynne, see page 32.

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Hawkins, T. S. Mercer, and C. L. Scott, Invert.

Reprod. Develop., 1993, 24, 243.

 3. J. R. Griffith, 1995, U. S. Patent 5,449,553, and

1997, U. S. Patent 5,593,732.

 4. R. E. Johnson and R. H. Dettre, J. Phys. Chem.,

1964, 68, 1744.

 5. J. K. Pike, T. Ho, and K. J. Wynne, Chem. Materials,

1996, 8, 856.

 6. A. E. Mera, J. K. Pike, M. Goodwin, and K. J.

Wynne, Polym. Prep. (Am. Chem. Soc., Div. Polym.

Chem.), 1996, 37(2), 765.

 7. A. H. Hogt, D. E. Gregonis, J. D. Andrade, S. W.

Kim, J. Dankert, and J. Feijen, J. Coll. Inter. Sci.,

1985, 106, 289.

 8. L. G. Britcher, D. C. Kehoe, J. G. Matisons, and A.

G. Swincer, Macromolecules, 1995, 28, 3110.

9. L. Lestel, H. Cheradame, and S. Boileau, Polymer,

1990, 31, 1154.

10. G. W. Swain, J. R. Griffith, J. D. Bultman, and H. L.

Vincent, Biofouling, 1990, 6, 105.

11. L. R. Ternoir, R. E. Foscante, and R. L. Gasmena, U.

S. Patent 5,275,645 (1994).
Four/1997 9

Introduction

The U.S. Navy has been searching for the ideal antifouling (AF) paint for steel hulls since the 19th century. The

Navy has long term service requirements - 5 to 7 years -

between drydocking and painting, and extended pierside time

where the majority of marine fouling occurs. There is a severe propulsive fuel penalty and possible compromise of the

ship’s mission if the hull is allowed to foul. Navy Formula

121, used by the fleet until recently, is a 70% cuprous oxide

by weight vinyl rosin based coating. This paint has long

been known to be inadequate for the service life requirements of Navy ships. Figure 1 shows the performance of a

population of both Pacific and Atlantic Fleet ships painted

with F 121. All ships in the populations had serious fouling

within either 24 or 28 months. The poor performance of F

121 provided incentive for research to develop improved

paints. Introduction of the organotin AF paints in the late

1970’s showed promise of achieving the Navy requirement

of 5-7 years unattended service between repainting in

drydock. Much effort was expended in research on

tri-organotin toxicants and multiple paint formulations with

these ingredients in the areas of performance and environmental effects. Field evaluations and ship trials were conducted on the many products that evolved from industry and

Navy research. However, the organotins eventually fell into

disfavor due to environmental and shipyard health concerns.

The commercial paint industry responded to the regulatory pressure on organotins by developing “tin-free” ablative AF paints containing cuprous oxide toxicant. The first

Navy test ship was painted with cuprous oxide ablative AF

paint in 1984. This ship, USS AINSWORTH (FF 1090) experienced 5 years of good performance from her AF paint

system. The rate of application of cuprous oxide ablative

AF paints to Navy ships accelerated after U.S. Navy organotin

AF paint use was prohibited by the U.S. Congress in 1986.

Although the ban was lifted in 1989, the U.S. Navy has chosen not to implement organotin AF paints. Instead, over 300

ships and submarines in the US fleet of 375 units have been

painted with cuprous oxide ablative AF paints from two primary commercial suppliers. A new military specification1

was issued covering AF materials.

The performance of the cuprous oxide ablative AF

paints has been generally satisfactory. While some ships

Biofouling, Fleet

Maintenance and

Operational Needs

Gerard S. Bohlander and Jean A. Montemarano, Naval Surface Warfare Center, Carderock Division, West

Bethesda, MD
10 Naval Research Reviews

have fouled before 5 years service, most have performed well,

providing performance exceeding that of the Navy Formula

121. All current AF paint applications utilize the ablative

AF coatings.

Cuprous Oxide Ablative

AF Paint: Performance

Analysis

The performance of cuprous oxide ablative AF paints

on U.S. Navy ships has been analyzed by collecting ship

inspection reports, conducting diver and drydock inspections,

and from a diving inspection data base maintained by the

Naval Sea Systems Command, Supervisor of Diving and

Salvage, Code OOC. This database has been developed from

inspection reports from commercial and Navy diving operations, and reports from underwater cleanings conducted on

cuprous oxide ablative AF paints. Paint condition and fouling ratings observations are based on visual and written guidance contained in a Navy document2

. Those familiar with

diver observations realize the difficult environment in which

divers operate. Visibility is often severely limited, and the

quality of observations may be dependent on the skill of individual divers. Thus, diver observations need to be verified by photographic documentation, but are valuable for

determining performance trends for these paints.

Figure 2 shows the age distribution for 170 USN Atlantic and Pacific Fleet ships painted with copper ablative

AF paint The bulk of the population was between 12 and 30

months old. The age distribution of ships that have had underwater cleaning is summarized in Fig. 3. About 25% of

ships painted with ablative AF paint have had underwater

cleaning. The fouling rating (FR) at the time of cleaning is

shown in Fig. 4. A FR of 40 or greater over 10% or more of

the hull is the criterion for underwater hull cleaning. Figure

5 combines the data in Figs. 2 and 3, and demonstrates a

slight increase in the number of fouled ships may be expected

as the population of copper ablative painted ships ages. Note

that the performance evaluation information is 4 years old,

so there are more ships painted today than reflected in this

analysis. Updated information is now being sought to improve this analysis.

Figure 1

The performance of Navy Formula 121 cuprous oxide AF paint

on selected Navy ships in the Pacific and Atlantic Fleets. This

shows the population of ships not requiring underwater hull

cleaning. Both fleets have all ships in the population requiring

cleaning due to marine fouling in 24-28 months.

Figure 2

The age distribution of the population of US Navy ships coated

with ablative cuprous oxide AF paint, as of May 1991. Both

Atlantic and Pacific Fleets are shown.

Figure 3

Performance of the population of US Navy ships painted with

ablative cuprous oxide AF paint. Graph shows number ofships

with marine fouling sufficient to require hull cleaning as a

function of time out of dock since painting.
Four/1997 11

Perhaps the most well known benefit from the use of

an effective AF paint is in the area of propulsive fuel savings. The accumulation of marine fouling on the hulls of

ships increases the hydrodynamic drag, requiring the expenditure of more horsepower to move ships through the water.

This increases propulsive fuel consumption. Ship trials have

documented reductions of from 10 to 60 % in propulsive

fuel consumption as a result of removing marine fouling from

a ship hull3

. The U.S. Navy propulsive fuel bill is approximately $500M/y, of which about $75M to $100M is expended

to overcome the increased drag to marine fouling. An effective AF paint could be expected to save a substantial portion

of that expense. The increasing use of ablative AF paints by

the U.S. Navy is expected to recover perhaps $35M to $50M

of the propulsive fuel increases due to fouling by providing

better performance.

A major additional advantage to an ablative AF paint

is the ability to modify the usual drydock paint maintenance

procedure. Normally, the underwater hulls of U.S. Navy

surface ships have been sandblasted and repainted at each

overhaul dry-docking, which occurs every 5 to 7 years. This

was done because the standard AF paint, F 121, becomes

depleted of cuprous oxide on it’s surface, accumulates a light

colored layer of insoluble copper salts, and loses its AF properties. Drydock paint replacement is also done to remove

and replace the anticorrosive (AC) paint, which is an epoxy

material. Ideally, the ablative AF paints do not become depleted of cuprous oxide on the surface of the paint if the ship

has had sea time sufficient to activate the ablation process.

Even if little ablation has taken place, it is possible to wash

the spent paint matrix, which is soft, from the surface of the

paint after entering drydock. Additional coats of AF paint

can then be applied. Thus, the option to retain the existing

paint system now exists. Significant cost savings, ranging

from $300K for destroyers to $1.5M for aircraft carriers, result from paint system retention. Also, this minimized the

generation of hazardous waste from the paint removal operation. In addition, it is now possible to defer drydockings

if they are being planned primarily for paint removal and

replacement, with significant cost avoidance. It is necessary, however, to have provisions to capture and contain the

washwater in drydock, since it will contain significant

amounts of cuprous oxide. While most Navy drydocks cannot contain liquid waste, progress is being made to modify

some major docks at naval shipyards to address this issue.

Operational Experience

Six U.S. Navy aircraft carriers have taken advantage

of the ability of the ablative AF paints to be retained at

drydocking. In each case, the ships had docked with a cuprous oxide ablative AF paint system on their hulls, ranging

in age from 18 to 60+ months of service.

The first ship, USS AMERICA, was to undergo an

overhaul in 1987 of one year in drydock followed by 5 years

of service prior to the next docking. It had been painted 18

months before, in April 1985, with the ablative AF, due to

severe marine fouling of the F121 AF paint. The repainting

had been done with the understanding that the ship would

not be blasted and repainted at the upcoming overhaul

drydocking. A new maintenance procedure was established

for the care of the ablative AF paint after docking. After

entering dock, the hull was hydrowashed with 2500 psi fresh

water to remove biofilm slimes and any surface contamination that would effect adhesion of the subsequent coats of

ablative AF paint. A dry film thickness (DFT) survey was

done after hydroblasting to characterize the thickness of the

existing paint. The DFT survey was done with electronic

DFT instruments based on magnetic induction. These instruments are capable of storing several thousand readings,

and downloading them to a laptop computer in the drydock.

Figure 4

Observations are made on Navy ships of the marine fouling

on the hull prior to underwater cleaning. The “Fouling Rating”

(FR) scale runs from 10 to 100 as fouling increases in severity.

This graph shows the reported FR distribution when copper

ablative AF paints were cleaned.

Figure 5

This is a combination of Figures 2 and 3 and shows the

distribution of ages of the population of ablative cuprous oxide

painted Navy ships and the population with serious marine

fouling.
12 Naval Research Reviews

The results of that survey, expressed as a histogram of frequency of occurrence, is shown in Fig. 6.

A DFT survey had been done when the ablative AF

paint was applied, and that is also shown as a second data set

in Fig. 6. There is a slight difference in the average and the

mode of the populations, indicating that some limited ablation had probably taken place in 18 months of service.

There was, however, a problem with the hydroblast

operation. Inexperienced operators had increased the pressure to 6000 psi, from the recommended 2500 psi, resulting

in some scaring and removal of the existing AF paint. This

necessitated the application of 3 coats of ablative AF paint

to ensure that sufficient thickness of paint was present over

the entire hull. Subsequent underwater surveys indicated

good AF performance with some intercoat adhesion failure

between the original and additional coats of ablative AF.

When a survey indicated calcareous fouling was present, a

hull cleaning was conducted. A cost avoidance of about

$1.4M was realized from deferring the complete removal

and repainting of the underwater hull paint system. The ship

was subsequently drydocked in 1992 for removal and replacement of the paint system after 8 years of satisfactory

service.

The second aircraft carrier, USS CARL VINSON, was

docked after 42 months of service on a copper ablative AF

paint. Instead of hydrowashing, this ship was given an underwater hull cleaning just prior to docking, and a fresh water wash with fire hoses immediately after docking. The

paint system was in excellent condition. One coat of AF

paint was applied to this ship in the fall of 1991, after all hull

cuts had been repaired and paint touchup work had been

completed.

An additional benefit of this process is the elimination

of the consumption and disposal of between 3 and 6 million

pounds of abrasive blasting material that would have been

used to remove the existing paint system. This amount is for

a CVN 68 class ship which has 160,000 sq. ft. of underwater

hull area. This abrasive, contaminated with copper waste,

costs from $25 to $650/ton to dispose of, depending on which

state controls the disposal site. Thus, the retention of the

existing paint system can save from $80,000 to $2,100,000

in waste disposal costs, in addition to the labor and material

cost savings from avoiding the blasting and total painting

operation. These costs, while quite variable, range from

$750,000 to $1,400,000. The costs of touchup of the various hull cuts and paint damaged areas must be subtracted

from this total.

A third ship, USS ENTERPRISE (CVN 65), painted

in 1986, was docked after 37 months of service on an ablative AF paint. The paint system was also in very good condition. The paint system was hydrowashed with 2500 psi.

fresh water, and the hull was overcoated with one coat of

ablative AF prior to undocking. This evolution saved

$1,600,000 in maintenance funds. It is expected that this

paint system will operate until 1999.

A fourth ship, USS NIMITZ (CVN 68), was originally

painted in 1992 with an epoxy AC and an ablative AF paint

system. The ship was again docked in 1994 after having

been in service 30 months with its existing ablative AF paint

system. That paint application had been monitored in

drydock and baseline DFT data taken. The paint was in excellent condition upon docking, and the decision was made

that it was unnecessary to overcoat the AF paint, based on

the comparison of the paint DFT data from the two dockings

(Fig. 7). NIMITZ is expected to dock in March of 1998. A

decision will be made at that time concerning retention or

replacement based on the future plans for the operation and

maintenance of the ship.

USS DWIGHT D EISENHOWER (CVN 69) presents

a unique situation for life cycle monitoring of ablative AF

paints. The ship was painted in 1985 with an ablative AF

paint system. Baseline DFT data was taken on both the AC

and AF applications. The ship drydocked in January 1993,

where another DFT survey was done (Fig. 8). There was a

Figure 6

The paint dry film thickness (DFT) is measured on the hulls

of Navy ships. This shows 2 populations of data taken 18

months apart on USS AMERICA. It demonstrates a slight

reduction in thickness of the ablative AF paint with time in

service.

Figure 7

This shows the underwater hull paint DFT distribution on USS

NIMITZ at undocking and 4 years later. It demonstrates a

reduction in thickness of the ablative AF paint.
Four/1997 13

10 mil decrease in the average paint thickness over the 8

years of service. It is likely that a significant amount of this

thickness reduction was due to two underwater hull cleanings

that took place prior to docking, though the paint system

was in excellent condition. The decision was made that no

overcoat was necessary, since the ship was scheduled to

drydock in late 1995. An additional set of DFT data will be

taken at that time to document the condition of the longest

paint system that has been monitored, a total of 10 years. It

is likely that the paint system will be removed and replaced

due to the length of time before the next planned drydock.

The life cycle of the anticorrosive paint system is also

very important, since keeping the AF paint in service will

also demand a service life in excess of 5 to 7 years for the

AC system. The USN uses either the military specification

MIL-P-24441 polyamide epoxy or several commercial epoxy paints for protection of the steel hull from corrosion.

Anticorrosive paint technology has been evolving towards

the use of fewer coats of paints that contain high solids materials, and thus may be applied in thicker but fewer coats. It

is expected that these materials will provide lengthened service life. The new maintenance procedures that allow retention of the AF paints will take better advantage of the

increased service life of these improved AC paints.

There has been a recent experience in which the retention of an ablative AF paint system was not possible. USS

THEODORE ROOSEVELT docked after 5 years of service

with about 3% blistering and corrosion scattered over the

hull. The wide distribution of the blistering would have required significant blasting for surface preparation. It would

not have been economical to touchup these scattered areas,

and the decision was made to remove and replace the entire

paint system at a cost of about $750,000.

Emerging New Coating

Technology for Fouling

Control

The use of current AF copper paints is coming under

increasing regulatory pressure which may leave the Navy

with no acceptable alternatives. The Navy sets the pace in

AF coating technology due to long term service requirements

- 5 to 7 years - in order to meet overhaul cycles and extended

pierside time where the majority of fouling occurs. New

emerging coating technologies must maintain the required

operational service life as restrictions on application and removal of copper increase. The development of new fouling

control methods has dual use applications. The military has

a requirement for environmentally safe AF hull coatings

which provide extended service life and increase intervals

between scheduled drydockings. In addition, environmentally safe AF coatings offer tremendous potential for application to the commercial shipping and pleasure craft industry.

This dual use application provides the necessary incentive

and introductory technology bridge to the commercial sector in order to establish industrial preparedness for coating

production for both military and commercial applications.

One approach addresses the novel concept of providing long term protection with non-polluting/biodegradable

AF coatings. These coatings are based on two concepts -

“easy release” technology and natural AF agents. In short

term screening tests, both concepts have demonstrated their

AF efficacy. Easy release coatings are based on the concept

of minimal surface energy which deters or negates the attachment of fouling organisms. Underwater husbandry will

be necessary for periodic removal of the fouling organisms.

For the optimal easy release coating, organisms that do attach will be removed by the passage of the ship through the

water. Natural antifoulants are compounds that are found

indigenous to marine organisms that prevent the attachment

of fouling organisms. Eelgrass and certain corals exhibit

this phenomena which may be attributed to naturally occurring “chemicals”, i.e. natural antifoulants. These natural

antifoulants biodegrade rapidly in seawater. Therefore in

order to be effectively delivered, these antifoulants may need

to be incorporated into controlled release systems. Efforts

are ongoing in the Navy community to develop microencapsulation based on a complex coacervation process to contain

the antifoulant in capsules which are then incorporated into

the coating matrix. These delivery systems will then be formulated into coatings which may couple the natural

antifoulant controlled release system with the easy release

coating matrix. Since some of the natural antifoulants are

complex biochemical compounds, simple chemical analogues are being sought which mimic their AF efficacy. Criteria from the Environmental Protection Agency will need

to be established for this new class of naturally occurring

Figure 8

This is the paint DFT history of the underwater hull of USS

DWIGHT D EISENHOWER. The ship was painted in 1985

with 2 coats of epoxy AC then 3 coats of ablative AF. It docked

in 1993 and the DFT measured showed a 10 mil decrease in

average thickness.
14 Naval Research Reviews

AF compounds. Active industrial participation is anticipated

in the manufacturing scale-up of both concepts as well as

the commercialization of these products. The final AF coating system will be renewable so that it can be recoated without blasting to bare metal.

Easy release coating technology is the more mature of

the two fouling control technologies with limited commercial products available. These products are normally silicone based elastomers which exude a silicone oil which

makes the settlement of marine organisms difficult. When

organisms do attach to these coatings they are easily removed.

The silicone topcoat elastomer is normally bonded to a traditional epoxy anticorrosive system by a tie coat. However,

silicone elastomers have poor toughness properties. The

Navy requires a durable, tough coating which will be able to

withstand the rigors of the operational environment. Under

exploratory development funds ONR has demonstrated the

feasibility of developing durable, tough materials with improved fouling release properties and a Navy patent has been

awarded. The marine coating industry is geared to the commercial market which normally dock at a 2-3 year interval.

Industrial participation projects will commercialize the Navy

products, and increase the competitive commercial source

base for these coatings. Utilizing non-toxic technical advances to biofouling control has medium to high technical

risk since these are unproven technical approaches for long

term operational performance.

A near term solution to provide 5-7 year unattended

service for Navy hulls is the incorporation of cobiocides into

copper ablative AF paint matrices. There are several

cobiocides commercially available. These are organic pesticides which are effective against a wide range of fouling

organisms. The cobiocides provide long term protection

when used in combination with cuprous oxide or other AF

agents. Once leached from the AF coating the cobiocide

degrades rapidly in seawater and is not persistent in the environment. Commercial marine coating companies offer

copper AF paints incorporating these cobiocides which are

currently in use in the world market; however, they do not

provide equivalent performance to an organotin based ablative paint4

. Underwater hull cleaning requirements will need

to be evaluated as well as environmental ramifications of

cobiocides release into the water column.

Hull Maintenance

While the introduction of effective anticorrosive paints

and improved ablative AF paints have contributed to increased readiness, reduced operational costs and extended

the time between drydocking, often repair work is not discovered until the ship is in drydock. The planning and execution of maintenance for hull systems is difficult and

expensive. Most Navy surface ships typically have docking

intervals from 5 to 7 years. Periodic diver inspections are

conducted for assessment of marine fouling on the hull, but

diver inspections are not always able to discern deterioration of the many components on the hull. Thus, it often happens that significant additional repairs beyond what has been

planned are discovered. Unplanned repair work is always

very expensive to conduct, both due to potential interruptions in work already planned, and due to additional costs

likely to be imposed by the shipyard for the unplanned work.

Thus, it is advantageous to have as much information as possible regarding the hull condition prior to docking.

In addition, the process of underwater hull cleaning

for marine fouling removal, when done on copper ablative

coatings, has the potential to discharge unacceptable amounts

of copper toxicant into Navy harbors. While the cleanings

conducted on copper ablative painted ships have been successful in removing fouling and reducing the propulsive fuel

penalty, the ablative paints are soft, and this may cause the

cleaning operation to remove from 1 to 2 mils of AF paint.

This does not effect the long term performance of these paints,

since they are typically applied about 15 mils thick. However, the paint debris discharged during the cleaning may

contain significant amounts of cuprous oxide, which then

enters the harbor. The underwater cleaning of ablative AF

paints was investigated by the Navy to quantify the copper

release, since it is quite possible that regulatory agencies

would question the cuprous oxide discharged during this

operation.

An Automated

Underwater Hull Vehicle

Concept

The challenge of monitoring the condition of the underwater hull systems during service combined with the

desire to reduce the potential discharge of copper compounds

into Navy harbors has resulted in the development of a concept for a tethered automated hull maintenance vehicle

(AHMV). This vehicle would be designed to perform a

variety of hull inspection tasks in addition to performing hull

cleaning operations that reduce the discharge of copper compounds. The AHMV is similar to a remotely operated vehicle (ROV), which is commonly used for a variety of

underwater inspection and work tasks. It is planned to develop the AHMV over the next four years. The various functions of the AHMV are shown in Fig. 9.

The AHMV will have a 3 dimensional computer assisted acoustic navigation system to direct it to any location

on the hull and program its movements and operations. The

vehicle is planned to have both free swimming and hull crawling capability. This will enable the AHMV to move around

the hull for positioning and to monitor ICCP fields, and still

be able to roll over the hull with wheels to conduct the fouling removal and hull inspection functions since the navigation system will be programmed with the hull shape. A
Four/1997 15

topside computer will control the movement and the deployment of various sensors while fouling is being detected and

removed.

The ability to detect and clean marine fouling from

ship hulls is a critical function of the AHMV. Optical, laser

and acoustic methods of image analysis have been investigated for fouling detection. Both optical and laser imaging

were found to be limited by turbidity of the water, so acoustic imaging was selected. A high frequency acoustic imaging system, now under development, will have an imaging

area of about 2 x 8 feet and will detect and determine the

density of the fouling, and activate a cleaning tool for fouling removal.

Three methods are being evaluated for fouling removal

efficiency and copper discharge: rotary brush, high pressure

waterjet, and acoustic energy. Rotary brushes on a variety

of underwater cleaning machines have been used to remove

fouling for over 15 years. They are effective, but will abrade

the ablative AF paint causing discharge of cuprous oxide

into the seawater. Rotary brushes would also release cuprous oxide into the water if used on the AHMV, but the

total amount discharged would be much less, since only areas covered with calcareous fouling would be cleaned, and

not the entire hull.

High pressure waterjets have been used for a variety

of cleaning tasks in the ship maintenance area. Development work is underway in the Navy on high pressure waterjet

paint removal5

.This method has been used underwater for

cleaning fouling from propellers and sea chests. Recent work

with waterjets has demonstrated the ability to improve efficiency and reduce water consumption with multiple nozzles

on a rotating head. While fouling removal with waterjets

has been demonstrated, the amount of cuprous oxide that

would be released is a function of pressure, nozzle size and

translation rate, and needs to be carefully investigated.

Recently, work has been ongoing on the use of pulsed

power devices to remove marine fouling from underwater

surfaces. This method uses a high voltage spark to produce

a concentrated energy pulse and remove fouling. This technique is being investigated for the AHMV because it may

produce minimal disruption of the ablative AF paint with

reduced cuprous oxide discharge.

It is planned to contain and capture any copper containing effluent generated by the cleaning tool on the AHMV.

Several preliminary designs have been generated for a variety of shrouds and pumping system to accomplish this task

and transport this water topside to a pierside facility. Commercial units are available to process the water to remove

particulate and dissolved copper and copper complexes from

the effluent stream. Ion exchange resins, dissolved air flotation and ultraflitration are being evaluated for this process.

Several sensors will be incorporated into the AHMV

to provide information on underwater hull systems. They

include sensors for paint thickness, hull electropotential and

hull plate thickness. These sensors will be deployed periodically as the vehicle transits the hull, and the data will be

transmitted by the vehicle umbilical to the topside computer.

The paint thickness sensor will provide data to determine the amount of AF paint that has ablated off the hull

during service. This, when compared with baseline paint

thickness data taken at undocking, will allow maintenance

planners to decide if the ship needs additional AF paint at

upcoming dockings, and how much to apply. Figure 6 shows

a histogram of the underwater hull paint system on an aircraft carrier. There are two populations of data, one at application and another taken at a drydocking 20 months later.

Some decrease in total paint thickness is noted. Similar data

will be generated by the AHMV.

The impressed current cathodic protection (ICCP) system, which is installed on most Navy ships, protects the underwater hull from corrosion using several permanent anodes

mounted on the hull, controlled by a reference cell. It reacts

to changes in the electropotential of the hull by putting electrical current into the water until the reference cell detects an

acceptable potential, usually -850 millivolts. While generally effective, there are anodes failures and areas of uneven

current distribution. A reference cell mounted on the AHMV

is able to read hull potential and aid in determining proper

anode functioning and the static electrical field distribution

over the entire hull. Several ROV deployments to measure

hull potential have been accomplished. Hull plate thickness

can also be taken with an underwater sensor to detect thin

areas of the hull that may need future maintenance.

An artist’s concept of the AHMV includes brush cleaning capability and shows (Fig. 10) the various components

of the vehicle, including sensors, thrusters, and cameras. A

block diagram of the various systems coupled to the topside

support electronics, which will provide programmed naviFigure 9

This is a block diagram of the various components of the

Automated Hull Maintenance Vehicle.
16 Naval Research Reviews

gation control, data analysis and video of the hull is shown

in Fig. 11.

A typical vehicle deployment sequence will involve

setup of an equipment van on the job site, and lowering the

AHMV into the water, as shown in Fig. 12. A record of the

areas cleaned and data generated by the various sensors will

be maintained and be available to assist planning necessary

maintenance in upcoming drydock availabilities, decreasing

costs and time in dock. Ultimately, the underwater hull system data generated by the AHMV can be transferred to robotic paint removal and application systems in drydock.

Finally, as shown in Fig. 13, data taken in drydock, such as

paint thickness, can be transferred back to the AHMV computers, thus providing a continuity of maintenance records

over the ship operating cycles, reducing costs and improving efficiency.

Conclusion

The U.S. Navy is faced with expensive maintenance

requirements to maintain the underwater hulls of ships free

of marine fouling and to provide fuel efficiency and mission

capability. Conventional cuprous oxide containing paints

did not meet performance goals, and have been superseded

by ablative matrix coatings that offer improved performance.

However, they still present issues of long term performance

and copper containment during waterborne and drydock

maintenance. A need exists to control marine fouling with

minimal or no release of toxicants into the marine environment. A new generation of fouling control coatings that are

free of heavy metals are now being developed. These materials function by discouraging fouling settling, and providing a poor surface for fouling adhesion. Automated

underwater vehicles offer promise in providing in-water hull

cleaning and maintenance monitoring for Navy ships to reduce maintenance costs. Future needs for hull maintenance

include non-toxic fouling control coatings that can be applied in the industrial environment and that will self-clean

with low speed ship movement, and will provide 5-7 year

service. These coating should be renewable for service out

to 15 years, and then be easily removable.

Figure 10

This is a concept of the AHMV configured with a brush cleaner

and various hull sensors and an imaging system.

Figure 11

The operation of the AHMV will follow this flow diagram of

events as a Navy hull is inspected and cleaned, if required.

Figure 12

A concept of the deployment of the AHMV for Navy ship hull

inspection and cleaning.
Four/1997 17

Biographies

Mr. Gerard S. Bohlander is a senior engineer in the

Paints & Processes Branch of the Naval Surface Warfare

Center Carderock Division where he spearheads the development of hull husbandry technology in order to extend the

service life of Navy coatings and to improve the overall performance of Navy ships. In particular, his experience and

expertise in coatings technology are utilized in transitioning

research and development to Fleet usage, which has been

recognized by his receipt of the Meritorious Civilian Service Award. Mr. Bohlander directs efforts to reduce underway fuel consumption of Navy ships by minimizing the

increase in the hydrodynamic drag of combatants due to service induced hull roughness. Currently, he leads an advanced

development program to automate hull husbandry.

Mrs. Jean A. Montemarano heads the Paints & Processes Branch of the Naval Surface Warfare Center Carderock

Division where she is responsible for coatings RDT&E for

navy surface combatants and submarines. Current focus of

the branch’s programs is extension of maintenance

periodicities with environmental compliance specifically in

the areas of preservation technology and underwater hull

husbandry. Her numerous technical accomplishments have

been acknowledged by the Center in her receiving the George

W. Melville Award for her efforts on water repellent coating

for submarine periscope headwindows. She has also received

the Navy Civilian Meritorious Award, primarily for her work

on the development of controlled release polymeric AF

paints. Her efforts in this area included technological feasibility and advanced development including industrial

scale-up.

REFERENCES

1. MIL-P-24647B. April 2 1991. Paint System, Anticorrosive and Antifouling, Ship Hull.

2. Naval Ships’ Technical Manual, Chapter 081. April

15, 1986. Waterborne Underwater Hull Cleaning of

Navy Ships.

3. Hundley, L. and Tsai, S-J. October 1991. The Use of

Propulsion Shaft Torque and Speed Measurements to

Improve Life Cycle Performance of U.S. Navy Ships,

In: Fleet Maintenance in the 21st Century, Joint

Symposium of Commander in Chief, U.S Atlantic

Fleet and American Society of Naval Engineers.

4. Kjaer E.B. 1992. Bioactive Materials for Antifouling

Coatings. Progress in Organic Coatings, 20: 339-352.

5. Bohlander, G.S. and Wheeler, R. April 1992.

Reducing Underwater Hull Paint Maintenance Costs

by Using Ablative Antifouling Coatings. Presented at

the 9th Inter-Naval Corrosion Conference, Monterey,

Ca.

6. Bohlander, G.S. October 1991. Biofilm Effects on

Drag: Measurements on Ships. Presented at the 3rd

International Conference on Polymers in a Marine

Environment, Institute of Marine Engineers, London.

7. Williams, J. and Rice, R., “Navy High-Pressure

Waterjet Closed-Loop Paint Stripping System”,

Presented at the 1995 Ship Production Symposium,

Society of Naval Architects and Marine Engineers,

Jan. 1995.

Figure 13

The flow diagram of information to be collected on Navy hull

condition during underwater and drydock maintenance. The

information will assist automated inspection and paint removal

and application processes.
18 Naval Research Reviews

Dr. James R. Griffith retired recently from the position of supervisory research chemist at the Naval Research

Laboratory in Washington, D.C., which he joined in 1955.

His research has emphasized polymer synthesis related to

materials of Navy interest, especially long-term programs in

fluoropolymers and fuel resistant elastomers. He is well

known for using polymeric materials as linings for large fuel

storage tanks. Nearly all of the land-based Navy tanks worldwide are lined with materials he and his group produced. He

has been named on 42 Navy-owned patents: the most recent

being the NRL Edison Patent Award of non-toxic antifouling systems in 1996. He has won eight major awards highlighted by the NRL Sigma Xi Award in Applied Research in

1976, the Hillebrand Award of the Chemical Society of Washington in 1979, the Federal Laboratory Consortium Award

in 1984 and the Secretary of the Navy’s Award for Distinguished Achievement in Science in 1991.

Profiles

in Science

James R. Griffith
Four/1997 19

I. Overview

A. Overview and Goals: Issues

in Materials for Biofouling

Resistance

The development of new materials for designed uses

is one seminal example of modern science and engineering;

it involves orchestrating a complex symphony of

multidisciplinary team members who can bring specialized

expertise to the problem. In the development of materials

which can inhibit or control bioadhesion, the understanding

of fundamentals of biofouling, key concepts in polymer

materials science and molecular biology must all be integrated for the successful development of a new generation

of minimal fouling or fouling resistant coatings. A basic

premise in such development is the understanding of the surface chemistry of the material which could minimize and

control adhesion of marine organisms to surfaces. The design of such coating materials must utilize the state of the art

in materials surface chemical analysis, and is in fact a critical determinant in the development of new methods to define the surface structure and composition of the coating

interface.

Of particular interests are developing and utilizing

methods which can quantitatively describe:

• the surface of the material in contact with its environment to understand and predict long term usage and degradation mechanisms

• the interactions of materials with adsorbed polysaccharide and protein films which form the basis for

bioadhesives

• developing standard materials for testing and comparison

and then:

• the synthetic design and analysis of new materials

based on concepts learned from these studies.

This paper will provide a short review and overview

New Technologies for

Surface Analysis to

Assess Biofouling

Resistance

Joseph A. Gardella, Jr., Department of Chemistry, SUNY Buffalo, Buffalo, NY

Kenneth J. Wynne, Office of Naval Research, Arlington, VA

Thomas J. McCarthy, Department of Polymer Science and Engineering, University of Massachusetts, Amherst, MA

John F. Rabolt, Department of Materials Science and Engineering, University of Delaware, Newark, DE

Georges Belfort, Department of Chemical Engineering, Rensselaer Polytechnic Institute, Troy, NY
20 Naval Research Reviews

of the advances in instrumentation, measurement science and

methodology which are driving the analysis of polymer surfaces and present a short description of advances in methodology which are driven by problems in polymer materials

surface chemistry. The use of these methods is critical in the

development of model systems and standard materials; a

selection of work sponsored by the ONR-MIMI program will

be described. Finally, recent ONR sponsored results in the

application of these methods towards the development of

model systems and actual candidate coating materials will

be highlighted.

B. Specific Problems in Polymer

Materials Surface Science

In response to the challenges outlined above, rapid

development of new spectroscopic and microscopic probes

and continuing evolution of more established methods [1-4]

are advancing the surface analysis of polymer materials and

their interactions in biological environments. A good example of how instrumentation development has led to better

applications in polymer surface science is to follow the

growth of studies using X-Ray Photoelectron Spectroscopy

(XPS) aka Electron Spectroscopy for Chemical Analysis

(ESCA). Of common surface science related methods, the

inherent analytical aspects of the method, e.g., chemical

bonding information, high surface sensitivity, low damage

and relative insensitivity to the insulating properties of the

sample have made especially useful for the analysis of polymeric materials. The quantitative analysis of surface species and the detection of impurities or modifications at the

surface have been heavily traversed areas of ESCA application for polymer surfaces. It is now rather routine to use

ESCA to obtain surface composition of polymers, to follow

processing steps and degradation chemistry. Many of these

more sophisticated applications [1] have been driven by

advances in instrumentation [3]. These developments also

mean the method can be used to study systems of greater

complexity.

Despite the power of ESCA, to improve the understanding of polymer surface chemistry, more information is

desired about surface structure with further sophistication,

at a higher level of precision. For example, detecting the

presence and concentration of surface active functional

groups is desirable, indeed achievable routinely with ESCA

(especially in concert with infrared or Raman vibrational

spectroscopy). Yet more desirable for applications in interfacial bonding important in coatings is the knowledge of

orientation and subsequent reactive availability of these

functional groups. Beyond functional group information,

Table 1

Characteristics of some spectroscopic techniques suitable for studying polymeric materials.
Four/1997 21

monomer arrangement along a chain in copolymers, and

intrachain interactions, again, as they are exemplified at

the surface, would be important information not easily available. In addition, macromolecular chain arrangement,

termination, branching and micromorphological information (i.e. domain size and distribution) are important in

determining sophisticated surface structure property relationships. Further, isotopic sensitivity for detection of labeling

experiments and diffusion between interfaces is very desirable. Molecular weight distributions at or near the surface

(in comparison with the average, bulk distribution) would

have major impact in many problems. Higher degrees of

spatial resolution are required, in all three dimensions, so

that buried interfaces may be precisely determined. Speed

of data acquisition for kinetic studies is a general desirable

characteristic of analytical methodology which is very limited in UHV surface analysis of polymers.

To approach this level of structural and reactivity information at surfaces and interfaces, evolution in methodology of established methods and development of new methods

must both be accomplished. It is unlikely that any single

surface analytical method can answer all questions about any

system, and this is especially true for the complexity of macromolecular materials.

Table 1 shows a comparison of the analytical characteristics of surface sensitive spectroscopic measurements for

polymer analysis. Of course, other spectroscopic methods

not accomplished in UHV and other analytical methods play

a continuing and increasing role in determining surface structure - property relationships for polymers.

In particular, measurements of surface forces have been

valuable in describing the fundamental interactions at materials interfaces. Thus, this review will attempt to provide an

overview of six methods being utilized for polymer surface

interactions, ESCA, Secondary Ion Mass Spectrometry

(SIMS), High Resolution Electron Energy Loss Spectrometry (HREELS), accomplished in UHV, optical vibrational

spectroscopic methods (infrared and Raman) and the surface forces measurement. Specific examples of how these

have been developed and applied will be presented, with a

final section highlighting recent advances in polymer synthesis, design and characterization utilizing the methods.

II. Introduction to the

Techniques

A. Spectroscopy in

UltraHighVacuum: ESCA, SIMS,

HREELS

One common characteristic of several of the surface

sensitive spectroscopies described below is that they all require a UHV environment. This is mandatory for the electron or ion beam used either to probe or analyze the surface,

and to keep the surface clean of any contamination from the

environment. The level of vacuum necessary is extreme,

typically 10-7 to 10-10 millibar pressure must be maintained.

i. Electron Spectroscopy for

Chemical Analysis (ESCA)

Recent reviews have stressed the basic instrumentation [2] and advances [3] which have allowed great strides

in this technique. The basic ESCA experiment requires a

source of X-Rays, a lens system to collect and focus

photoemitted electrons, and energy analyzer and a detector.

This equipment, within vacuum, is generally interfaced to

data acquisition and analysis software on an operating computer. The desirable characteristics of the X-Ray source involve narrow line width, high intensity and minimal damage.

The lens system serves to allow collection of electrons from

a sample distanced away from the entrance of the analyzer.

This means other techniques can be focussed at the same

surface position. Further, the lens system usually retards the

energy of the incoming electrons, to provide higher energy

resolution. The energy analyzer operates as a filter to pass

specific energies of electrons to the detector. The detector

serves to amplify the electron signal and produce a counting

rate. Data acquisition using common interfaces allows for

signal averaging needed to provide acceptable signal/noise

ratios. In addition, the scanning and control of other parameters is possible with the computer.

Experimentally, a sample is introduced into the vacuum

chamber, illuminated by X-Rays and the electrons energy

analyzed. Commonly, it is thought that the vacuum will have

some effect on the sample, making the relevance to a “real”

environment questionable. Certainly, physically adsorbed

gases or liquids (e.g. water) are desorbed under UHV. Thus,

“wet” samples are difficult to analyze without special precautions. The use of freeze drying and low temperature

sample handling can mediate some of this problem [5] and

has been important in the development of methods for the

analysis of nonfouling coatings. Never the less, the UHV

can affect the sample in ways which prevent analysis of

samples in the ambient environment. It is notable that both

air and vacuum are very high energy environments, encouraging low energy species to segregate at the interface.

Vacuum effects, themselves are not always deleterious. In

many older ESCA instruments, for instance, sample heating

from secondary electron emission of the source causes sample

damage sometimes ascribed to vacuum alone. It is in the

best interest of the analyst to evaluate the range of damage

under any type of UHV analysis.

Typically, a low energy resolution survey scan allows

for determination of detectable elements. These signals are

then scanned at higher resolution conditions to provide information on binding energy and intensity. The binding en-
22 Naval Research Reviews

ergy of the electron gives chemical information, the intensity allows for quantitative analysis. Experimentally, it is

desirable to vary the angle of the sample plane with respect

to the analyzer. This allows analysis at different sampling

depths (known as angle dependent ESCA). The view of composition as a function of sampling depth is important in understanding the phase or compositional separation of

microstructures near surfaces. Further, the use of several XRay energies also produces different sampling depths. The

range of depths probed by the technique using the angle and

energy dependent experiments ranges from 10 to 200 ≈ in

polymers [2].

The most recent advances have been in three areas [2].

These are higher energy resolution for finer ability to distinguish chemical states, higher spatial resolution for lateral

analysis, and speed of data acquisition for studies requiring

low damage or kinetics. Also, the latter characteristic has

allowed for development of more dependable quantitative

information from polymers. Damage [4] has been a major

concern under the X-Ray, making quantitative analysis less

precise. Precision, accuracy and speed are now such that

polymer surfaces can be quantitatively analyzed in a few

moments of exposure. This allows more signal intensive

experiments to be accomplished. For instance, the analysis

of the topmost surface with a very shallow sampling depth

can be achieved by analyzing at very shallow angles. But,

this involves more signal to noise since very small signal

levels result. Modern instrumentation allows this to be done

without concern for sample damage. Alternatively, more

sensitive polymers like PVC can be probed and radiation

damage can be limited [4b].

The lower damage rates, the higher speed and the better signal/noise in modern ESCA machines have opened up

a better understanding of the limitations of previous applications of ESCA toward polymers.

ii. Secondary Ion Mass Spectrometry

(SIMS)

Secondary ion mass spectrometry (SIMS) describes a

“family” of very powerful surface analytical techniques [6].

Experimentally the SIMS technique involves focusing a primary ion beam of 0.5-30 keV energy onto a sample surface

and then extracting the secondary ions that are generated.

These secondary ions are separated, according to mass to

charge ratio, in a mass analyzer [7]. After being mass separated, secondary ions come in contact with an ion multiplier

where a signal is generated. In SIMS the primary ions usually consist of ionized Ar, Xe, Kr, O, Cs, or Ga atoms. Provisions are usually made for focussing /defocussing and

rastering the primary ion beam, and the secondary ions are

extracted by an electrostatic focussing lens arrangement.

Ultra high vacuum conditions (pressures less than 1x10-8 torr)

are necessary to ensure the efficient generation and detection of secondary ion species. In some instances, focussing

lenses are set up to provide a kinetic energy filter so that

organic secondary ions may be collected more efficiently.

Mass analysis is commonly accomplished using quadruple

mass filter (QMF), time of flight (TOF), or magnetic sector

mass analyzers. Instrumentally, the development and application of TOF-SIMS is an area of great current interest, since

TOF mass analysis gives the ability to detect ions of very

high mass, and therefore analyze high molecular weight

samples (10,000 to 20,000 amu) [8]. TOF-SIMS also has

very low limits of detection due to the quasi- simultaneous

detection of all masses.

The SIMS experiment can be performed with two different modes, namely “dynamic” and “static” [6]. In the

dynamic mode, high primary ion current densities, greater

than 1μa/cm2

, are used to gain elemental information as a

function of sputter erosion depth into the bulk. In addition,

elemental maps or images can be obtained by focusing the

ion beam to small (1-3μm) spot sizes (the so-called ion microprobe) or by illuminating the sample with a large- diameter ion beam and using ion optics to preserve the spatial

resolution (the ion microscope). In the dynamic mode, virtually all molecular information is lost due to the high current densities used. In the static mode, low (primary ion)

current densities and ion dosages (less than 1 nA/cm2

 and

1x1012 ions/cm2, respectively) yield a very small probability

that a primary ion will strike the same molecule or surface

region twice in a typical analysis time. Therefore, molecular structure and bonding information can be preserved.

“Static” SIMS was first described in 1970 by

Benninghoven [9]. With current density lowered to less than

10 nA/cm2, and defocussed ion beams, molecular secondary

ions were obtained for organic carboxylic acid compounds.

In 1976-1977 static SIMS spectra of 15 amino acids [10,11]

displaying parentlike molecular ions were published, causing increased interest in the technique. From that time, static

SIMS has been used to analyze a wide variety of synthetic

polymers [8, 12-17], biomolecules [18], and pharmaceuticals [19]. The focus of a majority of these studies was to

investigate secondary ion emission mechanisms and/or show

the capability of static SIMS to provide mass spectra of large,

thermally labile, organic compounds.

As larger molecules are probed from surfaces, the problems of predicting molecular ion formation mechanisms complicate spectral interpretation. The understanding of SIMS

generation of molecular ions from surfaces and [20] in

biomolecular mass spectrometry [21,22] is based on the close

relationship of the SIMS experiment to fast atom bombardment mass spectrometry [FAB or liquid SIMS] [23], plasma

desorption mass spectrometry (PDMS) [24] and laser desorption mass spectrometry [25]. These methods have been

particularly useful for the mass spectrometry of high molecular weight biological molecules, allowing molecular

weight determination, identification, and attempts at nucleotide and protein sequencing within the mass spectrometer.

It is important to note that a major difference between
Four/1997 23

SIMS and the other mass spectrometric experiments noted

is the surface sensitivity of SIMS. Thus, these other “desorption /ionization” [26] methods, while having distinct

advantages in signal/noise and ability to generate high mass

ions over SIMS [24b], are not appropriate for much research

in surface science.

In SIMS, the basic principles of ion formation for

atomic species have now been recognized as the same for

molecular secondary ions. This allows the derivation of

molecular structure information based on molecular ion and

fragmentation information similar to that inherent in traditional mass spectrometry. Along with such surface specific

molecular structure analysis, the excellent detection limits

and ability to simultaneously detect signals from multiple

species at very different concentrations, which is inherent to

mass spectrometry, has allowed sophisticated methods and

applications to mixture analysis. For organic and polymeric

materials analysis, this could include analysis of additives,

residual monomers, and other smaller molecular species, with

low detection limits.

iii. High Resolution Electron Energy

Loss Spectrometry (HREELS) for

Vibrational Analysis

Low energy electrons (1 - 15 eV) impinging on a solid

may excite electronic transitions or vibrational modes within

chemical bonds near or at the surface. This interaction causes

the electrons to lose energy in discrete amounts corresponding to the energy of the transition or excitation involved.

This is the basis of high resolution electron energy loss spectrometry (HREELS) [27]. As HREELS experiments have

been applied to molecular and polymeric surfaces with increased chemical complexity [28-33], the sampling depth

[the depth of excitation] and the information depth (the depth

from which signals originate] have become important issues.

Various empirical results have suggested the information

depth may be as shallow as 2-5D to as deep as 20 D [28-31].

A critical aspect is the burgeoning understanding of vibrational excitation mechanisms in HREELS

The interactions between the primary electrons and

the surface species leading to vibrational energy losses can

occur by three mechanisms: dipole scattering, impact scattering and negative ion resonance [34]. These mechanisms

determine the selection rules operating in the HREELS experiment. Dipole scattering is a long range interaction between the primary electron and the sample, on the order of

60 - 100 ≈. This mechanism is dominant when the experiment is run in the specular geometry. This selection rule is

the same as for infrared external reflection spectroscopies

[32]. When a dipole active mode is excited there is a change

in dipole moment, and the vibrational mode is observed. In

optical reflection spectroscopies with polarized radiation,

when the dipole moment in a species is parallel to a metallic

substrate, local electric fields are cancelled and vibrational

excitation is not observed. Where the dipole moment is perpendicular to the substrate, the local electric fields are

strengthened and the excitation is observed [27,32]. This

“surface selection rule”, common for optical vibrational spectroscopies at metal interfaces, also applies to HREELS used

to study adsorbates on a metal [27].

So called impact scattering is more prevalent when

the experiment is run off specular or when higher energy

primary electrons are used [27]. This mechanism is short

range, on the order of a few ≈’s. In this event, all vibrations

in a molecule can be excited. The cross section for scattering changes with the geometry of the experiment (specular

vs off specular] [27]. The electrons inelastically scattered

by this mechanism are broadly scattered such that fewer

electrons are collected in the small acceptance angle of the

analyzer. This diminishes the intensity of features due to

impact scattering, as compared to the higher intensity of the

dipole excited vibrations in the specular direction.

Resonance mechanisms were not reported in studies

of small molecules adsorbed onto single crystal metals previous to 1982 [27,p.10]. Resonance or negative ion resonance occurs when the primary electron energy corresponds

to the energy of an unoccupied molecular orbital (MO) (e.g.

the Lowest Unoccupied Molecular Orbital (LUMO)) of the

adsorbed or surface molecules. This allows primary electrons to be temporarily trapped in the MO. The captured

electron can remain in resonance for 10-14 to 10-12 seconds.

The exit of the electron from the MO causes excitation of

vibrations which enhances the loss features due to that vibration [27,33]. Vibrational enhancement by resonance only

occurs if the symmetry of the resonant state is compatible

with the vibrational mode [35]: the shape of the intermolecular bond which captures the electron is changed as the

electron leaves. When this shape change corresponds to a

vibrational mode, enhancement occurs. Vibrational modes

which already result from dipole scattering may be enhanced

by this mechanism. Changing the primary electron energy,

so as to be in resonance with the unoccupied MO, will affect

the loss features if they are enhanced by negative ion resonance.

While resonance enhancement of adsorbed molecules

and thin films has been reported [31, 33,36], many questions are still unanswered regarding this mechanism. When

resonance will occur is not firmly understood. Resonance

enhancement of vibrations from functional groups such as

CH3

 and CH2 has been observed in different materials

[31,33,35,37]. Whether the observation of enhancement of

the same functionalities in several different compounds indicates that resonance enhancement can be predicted in similar cases is not known for certain. How resonance is affected

by a material with many and various types of functional

groups (for example: polymers or organic thin films) is a

related concern. The extent or degree of enhancement for

any given functional group or material is also not well understood. Resonance enhancement in adsorbed films is an
24 Naval Research Reviews

open research area with several groups working on the issues of concern [29,31,33-37].

If a firm understanding of the contributions of these

mechanisms to band intensity can be developed with predictive capability, the sensitivity and selectivity of HREELS

for organic, or polymer thin film analysis would be improved.

For instance, the ability to selectively excite particular functional groups through resonance enhancement would allow

less complication for identifying structures within a complex spectrum comprised of signals from functional groups

of less interest (e.g. CH3 and CH2). This high sensitivity for

the topmost surface may be exploited, for example, in the

analysis of polymeric endgroup effects at surfaces [38].

B. Optical Vibrational

Spectroscopy: FT-IR and Raman

Methods

Vibrational spectroscopy has made an enormous leap

in sensitivity over the last decade driven mainly by users

who demanded the capabilities to investigate thinner and

thinner films [39]. Polarized FTIR spectra using grazing

incidence reflection from metallic surfaces can now be routinely obtained on films as thin as 15 angstroms with exceptionally good signal to noise ratios. With this kind of

sensitivity it now becomes plausible to investigate the orientation and order of an adlayer of protein on the surface of

a molecularly designed organic surface. Using the polarized nature of the infrared light it should be possible to assess the nature of the orientation of particular molecular

groups and then determine their role in the adhesive process.

In a similar way Raman spectroscopy could be utilized to investigate proteins adsorbed on organic surfaces

but, as yet, the sensitivity is somewhat lacking due to the

fact that the Raman scattering cross section of a molecule is

8-10 orders of magnitude weaker than its absorption

cross-section for the same normal mode. Recently however

with the introduction of CCD Raman detectors which have

quantum efficiencies approaching 80%, it should now be

possible to investigate thin films and adlayers with this powerful technique which has been used very effectively to study

protein conformation in both the solid state and in solution.

The inherent advantage of Raman over FTIR is the

non-destructive nature of the former and the lack of specific

sample preparation required. In the future the increased sensitivity of CCDs in the red will provide the ability to use

near IR lasers for Raman excitation thereby providing a

means to avoid the fluorescence usually associated with small

amounts of impurities which are found in native proteins.

Hence fluorescent free Raman will then be routinely obtained.

All in all, the future for the role of vibrational spectroscopy

in unraveling the physics of protein adsorption and adhesion

is exceptionally promising.

C. Surface Forces Measurements

Recent instrumental developments have allowed the

measurement of the noncontact forces between molecules

with the surface forces apparatus (SFA) and with an atomic

force microscope (AFM). The SFA is a method which allows the direct measure of the molecular or atomic forces

between surfaces [40]. With the SFA, the forces between

the molecules on two opposing macroscopic surfaces are

measured as a function of their separation distance; consequently, the full force law governing the material interactions is obtained directly [40]. The intersurface spacing is

determined with single angstrom resolution by optical interferometry, and the force is measured with 0.01 nN resolution by measurement of the deflection in a sensitive spring

supporting one of the surfaces. Thus, this resolution in both

the force and distance measurements make possible the direct determination of the molecular forces governing molecular and surface interactions.

Atomic Force Microscopy (AFM) can be used to measure intermolecular forces between a silica sphere (attached

to a cantilever tip) and a polished silica plate in aqueous electrolyte solutions [41]. In recent developments related to

the general problems associated with adsorbed polymeric

species, Biggs and coworkers [42,43] have also studied the

steric and bridging forces between surfaces bearing adsorbed

polyacrylic acid. A silinized silica sphere and plate have

also been used with AFM to obtain hydrophobic forces [44].

Gaub et. al.[45] have measured the intermolecular forces and

energies between ligands and receptors. Besides intermolecular forces, the predominant use of AFM is for imaging

surfaces to obtain information about topology [46].

III. Advances in

Methodology

A. ESCA

Three aspects of ESCA make it a desirable surface

analytical technique for polymers. Qualitative information

from chemical shifts allows the identification of surface functionality in a polymer. Quantitative analysis of the surface

allows the determination of elemental and functional group

concentrations. Finally, angle and energy dependent measurements allow this information to be determined for an indepth profile; albeit with great difficulty due to the attenuation

of signals from deeper depths. All of these aspects have

been discussed in great detail for polymers previously

[2,3,5,7,8]. However, the speed of newer instrumentation

increases confidence in the measurement because of the lower

amount of experimentally induced sample damage [7]. This

allows extension to more complex samples. Further, this
Four/1997 25

lower damage allows for higher accuracy in the measurement.

A major advance sponsored by ONR work [47-49], is

the development of a method to extract the structure of polymer microdomains as a function of depth. In particular, it

involves the transformation of concentration measurements

taken as a function of sampling angle, from different depths,

into a concentration depth profile. It is important to understand that the typical angle dependent ESCA results [50-53]

allow for a quantitative analysis of the weight percentage of

siloxane material (or any other component save trace impurities) at the topmost surface. The angle dependent experiments also give an indirect view of the change in composition

as a function of depth over the topmost 100 D. This result is

not a direct “in depth profile” of composition as a function

of depth because of the effects of attenuation of the ESCA

signal which confounds the result into an integral measurement. To recover the “in depth profile”, deconvolution of

the signal must be accomplished; previous work of this sort

on polymers has suffered from the lack of knowledge of copolymer structure. The recent ONR sponsored work utilized

a new approach to numerical solution of the deconvolution

problem and resulted in depth profile data to be recovered.

Typical examples are given in the final section on applications.

B. SIMS

Advances in SIMS methodology have been greatly

accelerated by the interplay of the use of rational model systems and the development of new instrumentation. For the

analysis of potential minimal fouling coatings, three specific

foci can be examined which impact on the development of

SIMS methodology. These constitute i) the model systems

for the conditioning film or a simulation of the polymer surface using structured well prepared thin organic films, ii) the

analysis of the “base” polymer materials themselves, and

iii) analysis using spatial information in two or three (depth

profile) dimensions. These will be reviewed separately.

i. The Study of Thin Organic Films

A critical factor to further use and understanding of

SIMS is improved ability to predict the results of analysis

using known features of ion formation mechanisms. Obviously, knowlege of both the presence and probability of ion

formation would be helpful in qualitative and quantitative

analysis. Prediction of the presence of particular ions from

molecular species has been complicated by the matrix sensitivity. Put in simple terms, this means that the presence of

a molecule in a particular environment does not guarantee

the formation of a particular kind of molecular ion. Thus,

much work is being accomplished to learn the surface chemistry which leads to the generation of simple molecular ion

species. Of the huge body of work on ion formation mechanisms in molecular static SIMS, much is focussed on model

systems using organic thin films on polycrystalline metal

foils.

Langmuir-Blodgett (LB) films have been now extensively used as model system to study the SIMS sampling

process for large organic molecules, to investigate secondary molecular ion formation mechanisms, and to develop a

quantitative method for static SIMS [53-62]. The use of LB

films as well-characterized model systems for SIMS analysis provides several advantages: (i) a highly regular and pure

model system can be prepared, (ii) molecular orientations

for large molecules are precisely determined, and (iii) modifications such as substrate changes, added cations, and most

importantly the “physical state” of the monolayer can be

changed by varying the applied surface tension during film

production [63, 64]. In addition, techniques such as optical

ellipsometry and reflection-adsorption infrared spectroscopy

can corroborate the nature of the film [64]. The detection of

molecular ions from a single (quantified) monomolecular

layer demonstrates the promise of the LB technique for producing a homogeneous system of large molecules. These

experiments illustrate the sensitivity of static SIMS to the

chemistry of the molecular species and substrate, the

“preionized state”, and the final emitted molecular ion. Some

of the studies have focussed on multilayer films produced

by LB methods, which is a distinct advantage of the LB approach over adsorption or self assembly, mainly to address

sampling depth issues [54, 55, 59, 60]. Methods to obtain

quantitative molecular ion data from static SIMS analysis of

LB films have also been published [57, 58]. The applications of SIMS to study self assembled monolayer (SAM)

films have been more recent, and have focussed on applying

SIMS to solve the chemistry of the films, rather than the use

of the films as model systems [65, 66]. This series of studies

illustrates the sensitivity of molecular ion formation to surface complexation and acid base chemistry, both seminal issues in adsorption of biological macromolecules.

ii. The Study of Polymers and

Polymer Surfaces

The first applications of the static SIMS to the analysis of polymer surfaces, low mass (0-150 amu) fragment ions

[12] were used to distinguish structure and isomerism (butyl

methacrylates) in a series of methacrylate polymers with different side chain structures via fragmentation pattern from

the sidechain, within a common pattern characteristic of the

backbone, primarily from positive ion spectra. A significant

result from these early studies was the differentiation of isomeric structures of butyl methacrylate, which could not be

distinguished by ESCA core level analysis and the analysis

of slight reactive surface degradations of the t-butyl methacrylate [67].

Subsequent improvement in SIMS experimental conditions, to lower surface damage induced by the primary ion
26 Naval Research Reviews

bombardment, has allowed for the analysis of higher mass

fragments that are related to the original structure of the polymer [14-16].

Applications of the TOF-SIMS experiment to study

high mass ions from nylons, polystyrenes, polydimethyl siloxanes and surface modified polymers [8,17,68,69] has presented information beyond characterization of the monomer

identity. For instance, high mass oligomeric ions generating

an accurate molecular weight distribution have been demonstrated [8,68]. The potential here is great; a surface sensitive molecular weight distribution would be extremely

important for many polymer processing problems. High mass

fragmentation patterns have been postulated to be sensitive

to intrachain bonding in nylons [8b] and composition in complex polyurethanes [70]. Differentiation of isomeric differences is evident from high mass ions also [8,68].

At masses in between the molecular weight distribution and the low mass fragmentation patterns from the monomer, ions generated from fragmentation along the chain into

charge stabilized ions can be detected. These are termed “nmer ions” [13]. These potentially have a great deal of information about intrachain bonding and chemical reaction sites.

N-mer ions have been observed from a variety of polymers:

teflon [17], homopolymer and copolymer Nylons, and segmented poly(ether urethanes) [15,71].

Focus on the mechanisms of formation of these three

types of ions involves different considerations, as the information content of the ion increases with mass. However,

the most current attention to the simple ability of static SIMS

to differentiate the surfaces of polymer materials with equivalent atomic compositions and difficult ESCA chemical shift

information has meant even great excitement about fragmentation pattern analysis.

Dynamic SIMS depth profile work has studied diffusion between isotopically labeled polymers [72]. In addition, a series of very exciting papers using dynamic SIMS

depth profiling on block copolymer thin films by Russell et.

al was recently described in overview [73]. From the monitoring of hydrogen vs. deuterium ion signals as a function of

sputter time, a regular series of domain structures was measured where one domain was deuterated. This study employed highly controlled block length block copolymers. This

work shows the power of focussing strictly on elemental ions

in determining profiles; where unique isotopic tags can be

used to identify one component, complex different ion formation mechanisms can be ignored. One nice aspect of the

work by Russell, et al., is the correlation to ESCA and light

and x-ray scattering. However, with these advances, much

work remains to be done to identify exact conditions which

describe sampling depth issues for ions other than atomic

ions, and instrumental advances to further enhance the probability of formation and detection of high mass ions.

iii. Imaging Static SIMS of Organic

Surface Chemistry

A recent application [74] was the use of imaging TOFSIMS to demonstrate the selective surface chemistry on lithographically modified fluoropolymers using silane self

assembly monolayer chemistry [75]. In this application, the

chemically sensitive ion imaging provided the only means

to contrast micron level modifications to the surface of a

fluoropolymer. The surface modification was accomplished

using masking techniques and plasma surface chemistry,

followed by direct silane reaction, which was then limited to

the lithographically modified regions. No electron microscopic or x-ray methods had the sensitivity to detect these

chemical changes, yet neural cell biological model systems

were quite sensitive to the spatial limitations. These results

demonstrate that the micron level spatially restricted monolayer surface chemical modification could be directly visualized by imaging unique ion signals from the fluoropolymer

and siloxane backbones. These materials provide wonderful test templates for examining biological adhesion to two

common surface chemistries: fluoropolymers and siloxanes!

These materials are being further developed under the ONR

program to test bacterial and marine organism attachment in

realistic environmental exposures.

This application of imaging for molecular species is

just beginning, but where spatially restricted surface chemistry is necessary, static SIMS promises to provide one of

the few chemically sensitive methods to complement scanning tunneling or atomic force microscopy of such films.

C. Surface Forces Apparatus

Within the laboratories of Professor Georges Belfort

at Rensselaer Polytechnic Institute, SFA measurements are

being applied to study the adsorption of exopolysaccharides

to polymer surfaces. The technique involves the direct measurements of the intermolecular forces between two separate layers; one, an adsorbed protein or exopolysaccharide

layer and the other a polymeric film. Each layer is deposited

onto a smooth mica half-cylinder. The forces are obtained

as a function of separation distance which is measured with

an interferometry technique [40]. Approaches are being

implemented similar to that utilized to elucidate the intermolecular forces and conformation of adsorbed molecules

between mica surfaces covered with surfactant [76], polymers [e.g. 77], lipid mono and bilayers [e.g. 78], and proteins [79-82] and glycolipids [83]. The thicknesses of these

films can be determined from the in-situ inteferometry. The

forces between the mica surfaces are obtained from a knowledge of the spring constant and the null distance without

forces between the two surfaces.
Four/1997 27

IV. Development of

Model Systems

Sponsored by ONR

A number of approaches have been supported by ONRMIMI towards the synthesis of model systems for polymer

surfaces with specific properties in the control of structure

in adsorbed biological macromolecules. A particular interest in several research groups is to develop surfaces with

multiple chemical environments. These systems are also

projected as potential coatings in contact with the entire

marine biological environment. Most of this work relies on

the development of polymer surface chemistry in concert

with the development of measurements which can describe

the particular structure and composition which effects particular macromolecular or marine biological outcomes.

Research led by Professor Thomas McCarthy at the

University of Massachusetts, Amherst, Department of Polymer Science and Engineering is part of fundamental work

on organic chemistry at polymer surfaces and interfaces (particularly aqueous solution – organic polymer interfaces). The

McCarthy group has developed (and continues to develop)

new techniques for introducing specific organic functionality onto the surfaces of chemically resistant polymers –

poly(chlorotrifluoroethylene), poly(tetrafluoroethylene),

poly(vinylidene fluoride), poly(ether ether ketone), polyethylene – particularly in manners that the depth of the modifications can be controlled. The techniques are normally

designed to place versatile and reactive centers in the outer

few tens of angstroms of the polymer samples. The McCarthy

research group studies the chemical reactivity of the surfaceconfined functionality and prepare series of samples with

known structure and measure properties (adhesion, adsorption, coefficient of fraction, wettability) and draw structureproperty correlations. Major current emphases are on (1)

the preparation of surface mixtures of functionality (both

random and patchy), (2) the adsorption of model polysaccharides (positively and negatively charged polyelectrolytes)

from aqueous solutions to surfaces of known structure and

(3) the modification of solid polymers by polymerization of

supercritical solution-infused monomers. The overall goal

of the research is to simultaneously understand the structure, properties and reactivity of polymer interfaces.

One approach to controlling the depth of incorporation of chemically reactive functional groups is to utilize

heterogeneous (solid/supercritical fluid solution) free radical polymerization of styrene in supercritical carbon dioxide

- swollen polymer films with the objective of preparing one

polymer within another matrix polymer. This method may

allow the control of the depth (into the film) of modification

and the gradient structure of the interface, however thus far

the conditions utilized (surprisingly) modify the entire polymer sample. Decompression followed by thermal initiation

using AIBN or tert-butyl perbenzoate yields polymer blends

with the polystyrene trapped inside the matrix polymer.

Polymerization prior to decompression yields more extensively modified products. Conditions have been studied that

render different concentrations of polystyrene throughout the

thickness of film samples of poly(chloro trifluoroethylene),

high density polyethylene, poly(4-methyl-1-pentene), nylon

66, poly(oxymethylene) and bisphenol A polycarbonate.

In the laboratories of Dr. John Rabolt of IBM Almaden

Research Laboratories, new thin film materials are being

synthesized to construct model polymer surfaces with various mixtures of controlled surface structure; the intent is to

provide these surfaces for challenges by adsorbed biological

macromolecules. Recently the synthesis of chemically

derivatized polymers which contain side chain functionalized

groups that promote chemisorption to metallic surfaces has

provided the motivation to investigate both the adsorption

kinetics from solution and topography produced during the

self-assembling process. Initial studies on sulfur derivatized

PMMA and poly(dimethyl siloxane) (PDMS) indicated that

the effect of side chain modification on the diffusion coefficient in solution was minimal and independent of the “sticker”

concentration. The surface morphology of the 30≈ films

produced was either glassy (PMMA) or rubbery (PDMS)

and quite porous raising the possibility of incorporating a

second component through either a co-deposition or sequential deposition process.

Using angular dependent XPS, contact angle,

ellipsometry and FTIR it has been possible to investigate the

deposition of a PMMA/PEO blend on a gold surface and the

sequential deposition of PMMA and F8-thiol on gold. Both

films are in the 20-30≈ range in thickness providing unique

heterogeneous surface domain structures. Shown in Figure

1 is the morphology that results after a PMMA-100 selfassembled film is immersed in an F8-thiol/ethanol solution

and then dried. This unique structure comes about due to

the porosity of the PMMA-100 film, the relatively short

length (17≈) of the F8-thiol and the tendency of F8-thiol to

form domains with its molecular axis oriented normal to the

surface.

PMMA-100 + F8-Thiol

30-35 Å

Figure 1

Schematic diagram of the morphology formed by sequential

deposition of PMMA-100 followed by dipping in a F8-thiol/

ethanol solution.
28 Naval Research Reviews

In a somewhat analogous fashion, Figure 2 shows the

resulting surface morphology when a PMMA-10/PEO blend

is co-assembled on gold.

Working in a relative concentration regime of the two

polymers such that a semicrystalline blend results, it is interesting to note that not only do crystalline PEO domains form

in this constrained geometry but that they are highly oriented with the PEO helical backbones normal to the substrate. Further co-deposition studies with other polymers

compatible with PMMA are in progress in order to determine whether this observation is a more general occurrence

in confined geometries.

In a related study, rubbery films of PDMS having various thiol side chain concentrations (1-3%, 5-10% and 100%)

were also self assembled on gold in order to access the role

of backbone mobility on surface topography. Preliminary

results indicate that due to the low Tg

., PDMS films are rubbery and still contain significant porosity. Angular dependent XPS indicates that nor all thiol groups are attached to

the surface as was found in the case of sulfur derivatized

PMMA. Studies of sequential deposition of F8-thiol in

PDMS films is currently in progress so as to further ascertain the role of surface heterogeneity on protein adhesion.

The modeling of charged adsorbed macromolecules is

under study in the McCarthy laboratory at Umass. This group

is undertaking controlled adsorption studies of polyelectrolytes to a range of surfaces using the layer-by-layer adsorption method pioneered by G. Decher. A range of uronic

acid-rich polysaccharides are adhesive to surfaces containing amines, but non-adhesive to other surfaces. Three model

amine-containing surfaces and control experiments indicated

that basic functionality is required for the adsorption of acidic

polysaccharides. Poly(allyl amine hydrochloride) adsorbs

spontaneously from aqueous solution to surfaces containing

carboxylic acids or even esters – poly(ethylene terephthalate) functions well as a substrate. Poly(sodium

styrenesulfonate) then can be adsorbed to the nascent

poly(allyl amine hydrochloride) layer and this surface functions as an adsorption substrate for poly(sodium

styrenesulfonate) then can be adsorbed to the nascent

poly(allyl amine hydrochloride) layer and this surface functions as an adsorption substrate for poly(sodium

styrenesulfonate) adsorption. This system provides two

model substrates (positively and negatively charged) for adhesion experiments and that poly(allyl amine hydrochloride)

and poly(sodium styrenesulfonate) are two useful model

adsorbers for substrate testing. Additionally early experiments indicate that the mechanical integrity of these

multilayers is high and may be the basis for an antifouling

coating.

V. Applications of

Analysis towards

Candidate Non-Fouling

Coatings

A. Assessing the near surface

structure and dynamics of new

multicomponent polymers

In the laboratories at SUNY Buffalo and the Naval

Research Laboratory, collaborations between research groups

headed by Professor J. Gardella and Drs. K. Wynne and T.

Ho, ongoing ONR sponsored work involves both application and development of ESCA and ToF-SIMS. Results

from the analysis of new polymer surfaces have lead to major breakthroughs in the development of these methods.

These new experimental results have provided insight into

fundamentals of phase separation and control of surface structure and composition through synthetic polymer design. The

development of the new methods to quantitative analysis of

polymer composition in depth using data recovered from

angle dependent ESCA has been critical to the understanding of the structure and projected stability of the films toward degradation or aging effects. A centerpiece to this work

was the systematic variation of bulk structure of the materials introduced in rational synthetic design. This provided a

critical impetus for the work.

The synthesis [47] of the series of structurally controlled siloxanes shown here, the variation of the siloxane

(the so-called soft block) block length and the composition

of the urea-urethane segmental block (the so-called hard

block) gives a rich field of materials to evaluate to learn what

structural or processing conditions controls success in surface properties (i.e. minimal fouling and stability). Both

PMMA-10 Oriened Ordered Cystalline

PEO Domains

Amorphour 20-25Å

PMMA-10/PEO

Blended

Region

Figure 2

Schematic diagram of oriented crystalline PEO domains

embedded in an amosphoase PMMA-10/PEO matrix.
Four/1997 29

composition and dimensional data of both the siloxane and

urea-urethane components can now be determined, allowing for a three dimensional picture of the composition of the

material near the surface. This means four characteristic

parameters can be recovered from ESCA data to describe

the surface chemistry of these materials. These are: i.) the

surface composition (which, in comparison to the bulk composition yields the surface excess concentration of siloxane),

ii.) the thickness of the surface excess layer, iii.) the composition of the subsurface urea-urethane rich region and iv.)

the thickness of the subsurface layer. The urethane rich phase

is simply the result of balancing the surface excess region of

pDMS.

The significance of the detection and quantitative description of the in-depth profile can be seen in Figures 3 and 4.

Both the structure, for example the block length of the siloxane, and processing, for example, annealing conditions, affect the resultant surface composition, and the in-depth

profile.

For example, comparing a series of polymers with similar composition but different average siloxane block lengths,

a pure siloxane surface (i.e. 100 weight % siloxane at the

surface) over the topmost 10 D only occurs with the longest

siloxane block lengths. This was a surprise, in that most of

the materials contain large amounts of siloxane (between 70-

96 weight %), yet shorter block lengths bring urea-urethane

to the surface. The role of the amount of urea-urethane in

the initial adhesive properties and the long term stability is

of concern. However, it was interesting to note that extreme

siloxane compositions and block lengths were necessary to

produce a pure surface, which is only one initial consideration for utility of the material.

Encouraging results were obtained when, rather than

annealing, more common coating processing conditions, differential solvent effects, were evaluated. It was shown that

materials with shorter block lengths of siloxane (and better

mechanical properties (vide infra)) would yield a pure siloxane surface after processing by annealing or more importantly, by judicious choice of solvent casting [48]. This

portends very well for the development of these materials as

potential coatings.

In related work on the same materials, Time of Flight

SIMS has offered a new approach to the extraction of segment length distributions near the surface. As mentioned in

the introduction, the ability to differentiate the molecular

weight of species at the surface would explain a number of

surface properties not predicted by compositional differences.

The approach used, as references, thin submonolayer preparations of the difunctional prepolymer pDMS and the resulting segmented PU-PDMS copolymer, using silver as a

substrate. This work shows that reasonable molecular weight

distribution data can be obtained. Figure 5 shows a comparison of the high mass results from ToF-SIMS analysis of

thin, submonolayer solution deposited films on silver. The

resulting peaks originate from series of ions (sometimes up

to four types of ions in series) in a pattern; the data must be

assigned a structural series, and then converted to intensity

values for one ion series which can represent the molecular weight distribution. In the case of the pDMS prepolymer

shown, with nominal average molecular weight of 1000 Da,

Figure 3

The Effect of Sample Processing on Depth Profile of Hard

and Soft Segments: Recovered In Depth Profiles obtained

from Angle Dependent ESCA Analysis of pU-pDMS

Segmented Copolymers. The volume fraction of N containing

polymer (i.e. the pU segment) is plotted versus the distance

from the surface, one escape depth is approximately 25D.

Curve A is for the pU-pDMS polymer with a 2400 nominal

molecular weight for the pDMS segment, as cast from THF;

curve B is for the same polymer after annealing at 120 EC for

15 minutes. Curve C is for the same polymer after annealing

at 120 EC for 2 hours.

Figure 4

The Effect of Hard Segment Structure on the Surface

Structure of pU-pDMS segmented Copolymers: Recovered

In Depth Profiles obtained from Angle Dependent ESCA

Analysis of pU-pDMS Segmented Copolymers. The volume

fraction of N containing polymer (i.e. the pU segment) is

plotted versus the distance from the surface, one escape

depth is approximately 25D. Curve A is for the pU-pDMS

polymer with a 10,000 nominal molecular weight for the pDMS

segment, as cast from THF; curve B is for the same polymer

after annealing. Curve C is for the 10, 000 MW pDMS

copolymerized with a longer pU hard segment cast from THF,

curve D for the same polymer after annealing.
30 Naval Research Reviews

the entire molecular weight distribution should be available

within the mass range presented. However, for the segmented

pU-pDMS copolymer shown in the lower trace, the ion series shown are ions due to fragmentation processes. Thus,

these cannot represent a molecular weight distribution; in

any case, the molecular weight of the copolymer is much

higher than the region scanned.

However, what is important is not the molecular weight

distribution of species in the copolymer, but the distribution

of siloxane segments near the surface.

Normally, the analysis of thick, continuous polymer

films, more representative of coatings involves sufficient

chain entanglement and intrachain interactions so as to mediate against high mass ions being generated [17]. In this

case, however, the volatility of the siloxane segment makes

it a prime candidate for generation of fragment ions from

the surface, despite solid state effects. High mass distributions from two different pU-pDMS copolymers were successfully measured where the segment length distribution of

the siloxanes was varied by using different pDMS

prepolymers [47]. In particular, polymers with average

pDMS molecular weights of 1000 and 2400 were subjected

to analysis, in the form of a solvent cast thick (ca. 100 um)

films. High mass ToF-SIMS results are shown in Figure 6.

Data was analyzed and the peak series assigned to the ion

series represented by the structure shown below. Since it

Figure 5

Comparison of ToF-SIMS High Mass Results from thin film

preparations of A) 1000 MW pDMS prepolymer and B) pUpDMS segmented copolymer with nominal 1000 MW pDMS

segments.

Figure 6

High Mass ToF-SIMS Results from thick films of pU-pDMS

copolymers: A) pDMS MW = 1000; B) pDMS MW = 2400.

(A)

(B)

(A)

(B)
Four/1997 31

contains the entire DMS segment, thus it can represent the

DMS segment length distribution at the surface.

These ions were assigned to the structure series shown

above. The resulting pDMS segment length distribution can

be obtained because these fragments contain the intact DMS

chain.

Figure 7 shows a comparison of the MW distribution

obtained from the pDMS oligomer analysis on the thin films

on silver (from spectrum in Figure 5a) to the segment length

distributions obtained from analysis of data in Figure 6. It is

evident that the change in molecular weight of the DMS does

not change the distribution of pDMS segment lengths at the

surface of the material. This means that the surface of the

polymer synthesized with nominal molecular weight 2400

should not have different properties expected from the longer

pDMS segments.

For the future of these applications there are numerous continuing challenges for the application of ESCA analytical methodology. Of course, the extension of this

methodology to other polymer materials, chosen as candidate coatings and synthesized by ONR sponsored researchers is underway, in particular, fluorocarbon urethane polymers

[49, 83, 84]. In addition, the ONR sponsored team has

worked to extend the measurement to evaluate the surface

compositional changes which may be due to marine exposure. To do this, recent efforts from SUNY Buffalo, sponsored by ONR-MIMI, have resulted in the development [5]

of a sample handling probe which allows polymer samples

in aqueous environments to be frozen below the glass transition temperature of the material, preserving the surface structure induced by the environment, for subsequent ESCA

analysis. This can allow ESCA data to be measured for simuFigure 7

Comparison of MW Distributions obtained from ToF-SIMS Data: A) MWD calculated for pDMS MW =1000 thin film analysis;

B) MWD calculated for pU-pDMS copolymer from thick film analysis pDMS MW = 1000; C) MWD calculated for pU-pDMS

copolymer from thick film analysis pDMS MW = 2400.

(A) (B)

(C)
32 Naval Research Reviews

lated marine exposures, to examine the long term structural

stability and allow a fundamental description of environmentally induced changes in these materials. Only two research

group in the world have developed such capability, and the

Buffalo group is the first to apply this to study simulated

marine exposures.

VI. Summary and

Conclusion

In this paper, a short overview of recent developments

in surface analysis of polymers is highlighted by the applications of these technologies to the development of minimal

fouling coatings. The challenges presented by the design

and synthesis of these new materials is a major driving force

in new approaches to the use of the data and its interpretation. These demands also are supported by advances in new

instrumentation and the design of model systems which are

relevant to polymer surface chemistry.

VII. Acknowledgments

The authors acknowledge support from the Office of

Naval Research Molecular Interactions at Marine Interfaces

program and the contributions of postdoctoral fellows and

graduate students who are supported by these funds.

Biographies

Dr. Joseph A. Gardella, Jr. was born and raised in Detroit, MI and graduated with a dual degree in Chemistry and

Philosophy from Oakland University in Michigan, then matriculating for graduate studies in Analytical Chemistry at

the University of Pittsburgh, where he received the Ph.D.

with Professor David Hercules. After postdoctoral research

with Professor E. M. Eyring at the University of Utah, he

moved to SUNY Buffalo where he has been since. He spent

one year as a program manager/visiting scientist at the National Science Foundation. He is currently Professor of

Chemistry and Biomaterials and Director of the Materials

Research Instrumentation Facility (MRIF) at SUNY Buffalo.

His research efforts focus on the surface chemistry of polymer materials, especially those designed for contact with the

environment and complex biological systems.

Dr. Kenneth J. Wynne, who earned his Ph.D. from the

University of Massachusetts in 1965, is Program Manager,

Organic and Polymeric Materials at the Office of Naval Research. Dr. Wynne’s research interests include polymer surface design and characterization, polymers with novel optical

properties, electronically conducting polymers and

preceramic polymers. He has published over 80 papers, has

co-edited three books, and has 10 patents. He is a member

of the Editorial Advisory Boards of Macromolecules, Journal of Applied Polymer Science, Journal of Inorganic and

Organometallic Polymers, and Polymers for Advanced Technologies.

Dr. Thomas J. McCarthy is currently Professor of Polymer Science & Engineering, Coprincipal Investigator for

Center for University of Massachusetts Industry Research

on Polymers (CUMIRP). Professor McCarthy received his

B.S. in Chemistry from the University of Massachusetts at

Amherst and the Ph.D. in Organic Chemistry from Massachusetts Institute of Technology, under the direction of

George Whitesides. He has been at UMass in Polymer Science and Engineering since 1982. His research interests include polymer surface science, polymer synthesis and

modification, reactions in supercritical fluid-swollen polymers, adsorption of polymers at interfaces, plasma polymerization, and the development of asymmetric gas separation

membranes.

Dr. John F. Rabolt is currently Professor Chair of the

Materials Science Program, at the University of Delaware.

Previously, he was associated with IBM as a Research Staff

Member at the IBM Almaden Research Center, where he

was also Co-Director of the NSF Center on Polymer Interfaces and Macromolecular Assemblies, a collaboration between Stanford University, IBM and the University of

California, Davis. Dr. Rabolt received his B.S. in Physics

from SUNY Oneonta, and the Ph.D from Southern Illinois

University in Molecular Physics. His research interests include polymer and molecular thin films, vibrational spectroscopy, polymer surface chemistry, physics and

engineering.

Dr. Georges Belfort, Professor of Chemical Engineering and Director of the Bioseparations Research Center, received his Ph.D. degree in 1972 and his M.S. degree in 1969

from the University of California at Irvine in Engineering,

his B.Sc. (Chemical Engineering) in 1963 from the University of Cape Town, Cape Town, South Africa. He has recently received the 1995 American Chemical Society Award

in Separation Science and Technology, sponsored by Rohm

and Haas Co. He is President of the North American Membrane Society, and was elected a fellow of the American Institute for Medical and Biological Engineering in 1994.

Professor Belfort is an active consultant to industry in the

USA, Europe and Japan on membrane technology and

bioseparations. He has edited and coedited three books and

written with colleagues and students 18 book chapters and

over 90 reviewed papers.

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Four/1997 35

Introduction

Some of the facts regarding the ease or difficulty with

which ships move through the water are intuitively obvious

while others require complex hydrodynamic insights. Among

the obvious facts is that a smooth surface moving parallel to

the aqueous medium presents less drag than a grossly rough

surface of the same dimensions. The tendency of marine

organisms to attach to surfaces and render smooth areas rapidly rough has plagued sea-faring nations throughout history. It is no exaggeration to say that modern merchant

marines and navies can not afford to leave port with heavily

fouled hulls.

A new factor which has caused a major perturbation in

traditional methods of dealing with marine fouling organisms may be called “environmental consciousness”. The

established technique to prevent fouling is to supply the water

near a hull surface with a sufficient concentration of a broad

spectrum toxicant to prevent colonization. The toxicant has

been supplied by leaching from the hull paint. As long as

there were relatively few ships in vast quantities of ocean,

this was a logical and rather effective technology, but with

the coming of millions of pleasure boats in the harbors of

the world in addition to large numbers of commercial vessels, the looming prospect of biologically dead coastal and

inland waters forced new thinking.

Since the introduction of *TeflonTM to the general public as an antistick coating for cooking utensils, would-be inventors have dreamed of coating the exterior hulls of ships

with substance so that “barnacles would fall right off”. Such

an antiadhesive coating is known as a “fouling release coating”, and, in principle, it has a number of very attractive

features such as absolutely no environmental degradation and

long, effective life. However, efforts to implement this concept in a practical manner is fraught with many difficulties,

and this paper is intended to outline such efforts at the Naval

Research Laboratory since the early 1970’s.

Factors Affecting Marine

Adhesion

Ultimately, the strength of the bond between as adhesive and any surface depends upon constitutional factors;

Exterior Hull

Coatings in

Transition:

Antifouling Paints

and Fouling Release

Coatings

James R. Griffith and John D. Bultman, Naval Research Laboratory, Washington, DC

*TeflonTM is a trademark of the DuPont Company
36 Naval Research Reviews

that is, the chemical compositions of the components.1 Less

basic considerations such as surface roughness which results

in a mechanical locking of adhesive to adhered may be important, also. The ability of an adhesive in liquid form to

wet a solid surface can have a dramatic effect upon subsequent bond strength, and this ability, or lack thereof, is also

controlled by constitutional factors.2

Very few classes of synthetic polymers hold any promise for being sufficiently antiadhesive that all marine organisms bond with minimal tenacity. The various species of

barnacles, which are arguably the most important fouling

organisms, can bond with great holding power to epoxies,

polyurethanes, alkyds, polyesters, polysulfides, and practically all other common coating types. Only the

fluoropolymers such as TeflonTM, and the silicones offer possibilities for practical coatings. Polymeric hydrocarbons such

as polyethylene and polypropylene hold some promise but

lack necessary convenience-of-use characteristics to be seriously considered.

The cement with which a barnacle attaches its basal

plate to a substrate has been studied extensively,3,4 and the

general belief that it is some sort of miracle glue notwithstanding, it is merely a specialized polypeptide composition

which owes as much to its unique manner of deposition as to

its constitutional makeup. The overall process is so efficient, however, that there is no known solid substance that

the barnacle can not adhere to with some degree of binding.

Also, the amount of adhesive strength required to sustain a

close packed field of barnacles to a surface is worrisomely

small. Extensive efforts have been made to put hard numbers on barnacle adhesive values.5,6 But the variabilities in

this effort which may include the general health of the individual organism and small mechanical imperfections in the

adhesive layer, among other factors, make it not so simple

as it may appear upon first consideration. A barnacle lifted

perpendicularly to the plane of the surface experiences a suction cup effect which is roughly proportional to the basal

area and the effort to slide a barnacle laterally poses questions of force distribution and possible torque leveraging.

Thus, the problem of getting at the basic data upon which to

mount efforts to make ultimate fouling release coatings are

sufficiently daunting before the synthetic polymer chemist

starts his chore.

Fluoropolymer Fouling

Release Coatings

A report published in the December 1973 issue of Naval

Research Reviews discussed the basic synthesis efforts on

what later became known as “functional fluoropolymers”.

These were heavily fluorinated epoxy and polyurethane

materials which were designed to add convenience to

TeflonTM-like substances. Shortly after the publication of

that article, NRL was requested to provide advice to the

Norfolk Naval Shipyard regarding the preservation of a

highly valued harbor tug of World War II vintage which was

deemed to be irreplaceable. The shipyard was informed that

a very heavily fluorinated paint had been developed which

we were certain would provide near ultimate corrosion protection to the hull because of its extremely low equilibrium

water absorption values and excellent physical properties

such as toughness, adhesion, and abrasion resistance. The

head of the Production Engineering Department at the shipyard sprayed the external underwater hull of this vessel, called

the Sea Mule. Thus, a TeflonTM-filled, fluorinated polyurethane paint intended to provide long-term corrosion protection introduced NRL to experimentation on the fouling release

concept. The shipyard could easily lift the Sea Mule from

the water, and a regular series of cleaning experiments were

performed during the following 13 years.

Although the Sea Mule coating performed its original

mission almost to perfection, it suffered drawbacks as a fouling release coating. The barnacles in Norfolk harbor were

able to roughen the surface with their sharp shell edges, and

the mature barnacles could adhere to the uneven paint with

excessive strength, disallowing easy removal. In the case of

this coating the damage was shallow and it was never

breached down to the unfluorinated underlying paint. However, the ability of the barnacle to express this type of action

is a serious threat to the fouling release concept and a substantial difference in the ability of various silicone coatings

to resist this intrusion has been observed. At present it is not

certain which physical factors such as toughness, deform

ability, coefficient of friction, tear resistances, etc. determine

the result. If there is a defect in the coating such that the

immature barnacle can achieve a solid attachment, any silicone will be underminded at that spot.

It was found during a decade of work on fluorinated

surfaces of other types that barnacles can adhere with surprising force to any such surface including that of TeflonTM.

Since critical surface tensions below 10 dynes/cm can be

attained with such materials, the lowest known, it remains a

mystery why they are not better than they are. Our best

present guess is that there is a highly localized polarity to

the carbon-fluorine bond which allows very polar groups in

barnacle cement to develop a close association over time.

Silicone Fouling Release

Coatings

Patents have been issued for silicone fouling release

coatings since the early 1970’s, and there is presently a substantial number of U.S. and foreign patents on this subject.8,9

The empirical observation is that certain silicones are the

most antiadhesive substances known for hard fouling organisms in general although this has not been adequately explained in basic constitutional terms and all silicones are by

no means equal in this property. Generally, the silicones
Four/1997 37

that are the most effective are weak, “cheesy” materials which

are easily torn and have very poor qualities to be paint-like

compositions. Most of the best commercially available materials were compounded to be molding compositions, and

they have excellent antiadhesive properties for all synthetic

casting resins, such as epoxies. Early efforts to use them as

fouling release surfaces normally involved heat-cured compositions or “wall paper” techniques in which sheets of a

preformed materials were adhesively bonded to substrates.

Neither method was sufficiently convenient for use as ship

hull coatings.

In addition to the weak nature of the best releasing

silicones, there was the nontrivial question of how an extremely antiadhesive substance could be bonded to a substrate with such tenacity that it could provide long term

service as a hull coating. Several early experiments at NRL

in which the hulls of small boats were coated suffered premature failure because the rubbery silicone releasing surfaces came off in sheets after brief cruising.

These inherent problems with the silicones were eventually addressed effectively with what has become known as

a “duplex” approach. Certain polymeric materials which

contain components in addition to silicone are exceptionally

tough and tear resistant. One of the best contains a styrenebutyl acrylate copolymer which is compatible with the silicone components and which serves as a very effective

reinforcing agent.10 This composition is not a good

antiadhesive top coating, but it is a near perfect transition

layer because of the strength of the bonding that can be developed with epoxy substrates and with releasing silicone

top coats. Its use in conjuction with the better top coats imparts a “pseudo toughness” to the entire system because the

top coat can only be removed destructively.

Many secondary problems are encountered in an effort to make silicone molding compounds into practical marine paints. These include such things as flow and drip

control, spray ability, gel and cure times, volatile organic

component (VOC) restrictions, repair and recoat consideration of cost since the silicones are among the most expensive synthetic materials on the general market for caulks,

sealants adhesives, etc.

Additives for Hull

Coatings

Various materials have been added to silicone hull coatings in order to enhance the antiadhesive properties or to

impart an antifouling capability to the fouling release layer.

The addition of an oil which slowly exudes to the surface

provides a thin, liquid layer to enhance antiadhesive properties, and methyphenyl silicone has been an effective additive.11 Extensive efforts have been made to identify naturally

antifouling agents found in various sea organisms in the hope

that an analog or a portion of their unusually complex molecular structure will prove to have efficacy as a coating component.12 This approach has the particularly attractive feature that the agents preventing fouling may not need to be

generally toxic. Organic toxicants13 usually referred to as

“co-biocides” can be used effectively in silicones to provide

a temporary antifouling capability which is backed up by a

fouling release nature. The antiadhesive outer layer is not

particularly hospitable to sea creatures by its physical nature, and the dose of an organic toxicant required to keep it

clean in the sea may be minimal as a consequence. In general, an organic toxicant for this purpose needs to be “broad

spectrum” in that it prevents settlement of plant or animal

species, and it also needs to degrade rapidly once it is free in

the sea to produce harmless products. Very favorable results

have been obtained recently at NRL using this approach and

test panels in Chesapeake Bay for a year have been effectively antifouling while retaining a strong fouling release

nature as the active ingredients became depleted.

Laboratory Simulation of

Barnacle Adhesion

The population of marine fouling organisms vary seasonally in much of the world and the numbers of a given

species are not constant from season to season. When a new

fouling release surface is developed, it is highly desirable to

know quickly whether or not an improvement in fouling release performance has been achieved, and the prospect of

waiting for months for the answer makes for slow and inefficient progress.

In an effort to address this problem, albeit not perfectly,

the concept of the “pseudobarnacle” was developed at NRL.

As alluded to above, the constitutional makeup of barnacle

cement is rather mundane and if the creature could exude a

two-component epoxy under his growing shell edge he would

very likely be bonded to surfaces with greater tenacity. Because of this belief, it was thought that a two-part epoxy

putty formed into a small ball and pressed onto a fouling

release test surface to give a barnacle-shaped mass, may, upon

curing, give an adhesive value that could correlate with field

results on barnacles. A preferred material for the test is a

product of the Dexter Corporation with the trade name

“EPOXI-PATCH” and the “1C WHITE” version has been

used most extensively. An extensive evaluation of this concept is underway at the Florida Institute of Technology and

the preliminary results indicate useful correlations at the

lower, more important end of the anti-adhesive scale.

Concluding Remarks

Significant progress has been made in the generation

of radically new exterior hull coatings for undersea surfaces

which are more “environmentally friendly” than traditional

antifouling paints. Whether or not these coatings will be
38 Naval Research Reviews

operationally ready in the near future depends upon a multitude of considerations not the least of which is a reluctance

to abandon established old ways for uncertain new ones.

Nevertheless, the last few years have seen a determined effort by NRL, with ONR support, to make such paints longlasting and effective.

Acknowledgments

This work was supported by the Office of Naval Research.

Biographies

Dr. James R. Griffith is a supervisory research chemist at the Naval Research Laboratory in Washington, DC.

He received the Ph.D. degree at the University of Maryland

where he studied polymer chemistry with Professor William

J. Baily. His research interests have emphasized polymer

synthesis related to materials interests of the Navy with longterm programs in fluoropolymers and fuel resistant elastomers.

Dr. John D. Bultman is a research chemist at the Naval Research Laboratory in Washington, DC. He received

the Ph.D. degree at Georgetown University in physical biochemistry. His research interests have included the surface

chemistry of proteinaceous materials, anti-submarine warfare problems, biodeterioration of materials in the marine

and terrestrial environments, the natural resistance of tropical hardwoods destroyers, and commercial utilization of

byproducts resulting from the isolation of natural rubber from

guayule.

REFERENCES

1. W.A. Zisman, Ind. and Eng. Chem., 55, 18 (1963).

2 W.A. Zisman, J. Paint Tech., 44, 41 (1972).

3. J.R. Saroyan, E. Linder, and C.A. Dooley, Proc. 2nd

Int. Congr. Marine Corrosion and Fouling, 495

(1968).

4. J.R. Saroyan, E. Linder, and C.A. Dooley, Biol.

Bull., 139, 333 (1970).

5. J.D. Bultman, J.R. Griffith, J.D. Bultman, and H.L.

Vincent, Biofouling, 6, 105 (1992).

6. G.W. Swain, J.R. Griffith, J.D. Bultman, and H.L.

Vincent, Biofouling, 6, 105 (1992).

7. J.R. Griffith, D.E. Field, Nav. Res. Reviews, 15,

(Dec. 1973).

8. W. Muller and L. Nowacki, U.S. Patent 3,702,778

(1972).

9. R.R. Brooks, U.S. Patent 5,017,322 (1991).

10. J.C. Getson, U.S. Patent 3,776,875 (1973).

11. A. Milne and M.E. Callow, Trans. I. Mar. E., 229

(1985).

12. N.M. Targett and W.R. Stochaj, Recent Developments in Biofouling Control, Oxford and IBH

Publishing Co., New Delhi, 221 (1994).

13. J.A. Dupont, U.S. Patent 4,127,687 (1978).
Four/1997 39

Background

Because of concerns about the toxicity of the biocides

used in antifouling coatings, a need has emerged for a

non-toxic approach to the control of marine fouling. Antifouling coatings based on copper oxide have been used for

many years to effectively control marine fouling. As copper

oxide and other biocides dissolve in the water, an exponential release rate is observed and less biocide is available over

time. (1) In the mid-1970’s a significant advance was made

with the organotin copolymer based self-polishing coatings

which provided five years or more of fouling control. These

copolymers contained tributyltin chemically bound to the

polymer backbone. During service, the tributyltin antifouling agent was released at a constant rate and the resulting

polymer backbone would erode or polish due to the motion

of the ship through the water and become progressively

smoother. This offered a great economic advantage in both

dry docking and fuel costs. Improved copper oxide based

coatings that also ablated or polished with time appeared in

the 1980’s; however, they did not provide the performance

level of the self-polishing tin copolymers. Because the ablative or self-polishing coatings erode during service almost

to the primer, they essentially released all of their biocide to

the aquatic environment.

In the early 1980’s, organotins were implicated in the

failure of shellfish crops occurring in northwestern France.

Since all of the problems were located within a short distance from marinas containing pleasure craft where many of

the hulls were painted with coatings containing tributyltins,

a link between the shellfish problem and the organotins was

quickly established. The control of organotin containing

coatings rapidly spread to other coastal European countries,

Japan, Australia and New Zealand and to the United States

in 1988 (2). These regulations pertain mostly to small boats

less than 83 feet (25 meters) in length and not to ocean going

ships. Also, the Antifouling Paint Control Act of 1988 effectively prohibited the use of the tributyltin self-polishing

Silicon Fouling

Release Coatings

Harold L. Vincent, HV Enterprises, Midland, MI

Gregory G. Bausch, Dow Corning Corporation, Midland, MI
40 Naval Research Reviews

copolymers from the United States Navy fleet. The result

was a change to the less toxic copper oxide ablative coatings, which do not provide the level of performance attained

by the organotin self-polishing coatings.

Now even the copper oxide paints are under suspicion

for causing water pollution and are banned in freshwaters in

Sweden (3). Several states are regulating the amount of copper oxide in local waters. Shipyards are having difficulties

in meeting these new regulations, especially when using

underwater cleaning which leads to the release of high concentrations of copper oxide.(4).

Fouling control coatings that do not rely upon a biocide have been known since the early 1970’s. These coatings are based upon the polydimethylsiloxane (PDMS)

backbone and offer a non-toxic approach to biofouling control. They are described as “fouling release coatings” and

rely upon the physical nature of the coating to retard the onset of fouling and provide easy release when fouling does

occur.

History of Silicones as

Release Coatings

The use of silicone materials based on

polydimethylsiloxane as release agents and coatings has been

known industrially for nearly 50 years. Fluid additives or

coatings based on PDMS have been used in a variety of release related applications including mold release agents

and coatings, bakeware coatings, paper release coatings, and

anti-deposition coatings.

The first reported use of silicones as fouling release

coatings was in 1972 when the Battelle Institute was assigned

a patent (5) on the use of PDMS elastomers to prevent fouling by barnacles and other marine organisms. The silicone

could be applied as a curable gum or pre-cured sheets, and

room temperature curing silicone elastomers could also be

employed. Immediately following this patent was another

by Hempel (6) who also covered both heat and room temperature curing silicone elastomers. They compared the adhesion of the marine organisms to both heat cured silicone

elastomers and a polytetrafluoroethylene surface and found

that algal fouling released more easily from the silicone and

that barnacles could be removed under the pressure of a water

jet only from the silicone elastomer.

Early work at International Paint, now a part of

Courtaulds, showed that superior results were obtained when

a phenylmethylpolysiloxane fluid was added to a room temperature curing PDMS elastomer (7). Little evidence of fouling was seen after 2.5 years of immersion. A later study

claimed 10 years of fouling resistance (8). However, it was

reported that the poor abrasion resistance and physical properties including tear strength of these coatings limited the

range of potential applications to situations where such characteristics were not of primary importance such as aquaculture, off-shore structures, piping systems and power plant

water intakes(9). Since the finding that the fouling resistance of PDMS elastomeric coatings could be increased by

the addition of a silicone fluid, many researchers have followed this approach using such fluids such as trimethylsilyl

endblocked PDMS, PDMS/polyoxyethylene copolymers,

mineral oils, petrolatum, polybutylene and others (10,11,12,

13,14,15).

Unique Properties of

PDMS

The release characteristics of polydimethylsiloxane

(PDMS) are derived from the unique molecular structure

relative to organic polymers. PDMS is a heterogeneous

molecule (Figures 1 and 2) with a very flexible backbone.

As a result the polymer chain can readily adapt to the lowest

surface energy configuration (16). The siloxane portion is

inorganic, a silicate, has high surface energy, high intermolecular forces, and strong absorptions. The methyl groups

are organic hydrocarbons with their accompanying inertness,

low surface energy, low intermolecular forces and hydrophobicity. The diagrams below show the relationship between the inorganic and organic nature of the PDMS chain.

However, while surface tension measurements show tightly

packed methyl groups at the PDMS/air interface, the conformation of a PDMS monolayer may vary at other interfacial regions, such as PDMS/water. Evidence of this behavior

Figure 1

2-dimensional view of PDMS at interface of air and a high

energy substrate.

Figure 2

3-dimensional model of silanol ended PDMS at air/metaloxide

interface.
Four/1997 41

is that PDMS has a lower interfacial tension against pure

water than that of corresponding non-polar hydrocarbons.

M. Owens (17) has described five fundamental characteristics of polydimethylsiloxane as:

1. The low surface energy in air (23 mN/m2) is a direct result of the low intermolecular forces between the methyl groups. These pendent methyl groups act as close packed

methyl at the PDMS/air interface with little effect from the

siloxane backbone with its considerable polarity (41% ionic).

2. The siloxane backbone has a unique flexibility

which results from several parameters, including Si-O bond

length, flat siloxane bond angles, partially ionic (non-directional) nature of the bond, and an alternating divalent group

in the backbone yielding increased spacing between corresponding Si substituent groups. PDMS with a Tg of 146K

(-127 C) has the most flexible backbone among the common

polymers. As a result, it can readily adapt the lowest surface

energy configuration. The close packing of the pendent

methyl groups at the PDMS/air interface is possible due to

this flexibility. Also, this same property can allow a PDMS

monolayer to have a different conformation at an interface

other than PDMS/air, where the dipole of the backbone plays

a more important role in the interaction. This conformation

change must be driven by an overall reduction in the energy

of the system.

3. The high bond energy of the siloxane bond (445 kJ

Mol-1) is significantly greater than that for a carbon-carbon

or carbon-oxygen bond, and is responsible for the oxidative

and thermal stability of PDMS.

4. The partially ionic nature (41% ionic) of the siloxane bond results from the large difference in electronegativity of the silicon and oxygen atoms. Thus, the flexibility of

the molecule can permit configurations which allow interaction of the backbone dipole with polar substrates.

5. The critical surface tension of wetting for PDMS

(24 mN/m) is higher than the liquid surface tension (21.8

mN/m) which means that PDMS will spread on its own absorbed film and is not autophobic unlike many other low

surface energy materials.

Fouling Control

Mechanisms of Silicone

Coatings

Many of the theories used to explain adhesion can be

inverted to explain release. Therefore, a release coating

should exhibit several features at the “release” interface, including: barrier to mechanical interlocking, prevention of

interdiffusion, poor adsorption and lack of reaction, low surface tension and thermodynamic work of separation, limited

or no electrostatic attraction, non-setting, and provision of a

weak boundary layer (18,19). At the same time, it must exhibit at least some of the opposite properties in order to

adhere to the substrate. Although the phenomena of marine

biofouling is very complex and diverse, it is useful to generalize the control of fouling by PDMS coatings as a “release”

application.

There are several theories relating to the mechanism

by which PDMS based coatings minimize the level of or

retard the on-set of biofouling, and once fouling occurs maintain an easily cleaned surface with low fouling adhesion. It

is generally accepted that the lower level or inhibition of

fouling is due to the low adhesion forces (easy release) of

the biofouling organisms. First, it is useful to distinguish

two different methods: (1) Minimum adhesion force due to

coating structure (barrier coating), primarily related to surface energy to minimize wetting, and rheological properties

for reduced or reversible diffusion, and (2) Improved release

via use of partially incompatible additives (weak boundary

layer).

Low Surface Tension

Over a decade of research resulted in the finding that

the degree of biological interaction with a surface varies with

its surface energy. This relationship which reflects all aspects of biofouling, depicts a minimal adhesion of biomass

to surfaces that are dominated by closely-packed methyl

groups or are highly hydroxylated. This minimum also varies depending on the media. but is somewhere in the range

of 20-30mN/m. (20,21) Polydimethylsiloxanes with critical

surface tensions ranging from 21-24 mN/m fall within this

range. The Good-Girifalco-Fowkes equation is applicable

to situations where all interactions at the interface are dominated by dispersion forces. Dexter (22), Schrader (23), and

Baier (24) have all suggested that this is applicable to the

situation of low surface energy solids in the marine environment, and thus the minimum defined experimentally correlates to the point where the dispersion component of the total

interfacial energy is zero.

Mobile Molecular Interface

As mentioned, PDMS based materials have a very low

Tg or very high molecular mobility even as high molecular

weight elastomers. Initial data suggests that the resulting

low modulus of PDMS based coatings at room temperature

may be related to the ability to release hard fouling organisms. The modulus of PDMS based elastomeric coatings

can be varied to provide improved release, but this is often

at the expense of coating toughness.

Micromosaic Surfaces

The addition of a mixture of hydrophilic and hydrophobic fluids to room temperature cure PDMS coatings results in a cured film containing the liquids dispersed as a

micro phase. These microphases of additives yield an over-
42 Naval Research Reviews

all coating of alternating hydrophilic/hydrophobic nature. A

relationship between the least amount of fouling and the

clearest micro phase separation was observed. No relationship between fouling performance and surface energy was

observed (25,26). However, this could be due to the presence of a thin layer of these incompatible polar and non-polar

fluids on the coating surface. These fluids also form a thin

boundary layer at the coating/water interface and the polar

fluids could be partially water soluble resulting in an ablative boundary layer.

Weak Boundary Layer-Fluid

Additives

The commercial PDMS based coatings contain either

a mixture of polar/non-polar fluid additives or non-polar fluid

additives. These fluids are also slightly incompatible in the

cured PDMS coating which provides a driving force for diffusion to the coating/water interface, and results in a thin

layer of fluid. This liquid interface could act as a weak boundary layer resulting in improved release of biofouling. However, the addition of these fluids to organic coatings does not

result in any long term enhancement of fouling performance

such as occurs with the PDMS coatings.

Weak Boundary Layer-Polar

Surface

When PDMS is exposed to a high energy surface such

as water, theory suggests that a thermodynamic driving force

will exist to minimize the interfacial tension between PDMS

and water by having a siloxane and silanol rich PDMS/water interface. By reducing the interfacial tension to very small

values the driving force for the fouling organisms to adsorb

on the surface has been greatly reduced. Also, due to the

weak water boundary layer on such a surface, it will be difficult for the fouling organism to displace water completely

and form a strong bond.

As Zisman and co-workers first indicated (16), an electrical attraction between the siloxane backbone and water is

expected because of the semi-ionic nature of the Si-0 bond.

Also, some residual SiOH can remain on or migrate to the

surface of the cured coating and these acidic sites can interact with counterions present in sea water. The ability of

PDMS to change conformation in response to its environment has been reported elsewhere (16,27,28,29). Underwater surface tension work at Dow Corning and the Florida

Institute of Technology is believed to be the first measurements of the reorientation of PDMS underwater (30). Additional data gathered at Dow Corning on commercial coatings

stored underwater in a sterile environment also show a decrease in the water contact angle over time. This change

could be due to siloxane backbone conformation change,

silanol species migration/orientation to the interface, or some

type of hydrolysis. This reorientation mechanism is not possible in organic polymers since their structure does not allow heterogeneous behavior.

Technology of PDMS

Coatings

Polydimethylsiloxane coatings have their roots in room

temperature curing silicone sealants such as the familiar

Bathtub Caulk (R). A sealant can be dispersed in solvents

such as xylene, toluene or mineral spirits and the cured coating will offer a degree of fouling resistance. However, the

high molecular weight of the PDMS used in the sealants results in a high viscosity coating which must be reduced with

sufficient solvent so that it does not meet VOC regulations.

A PDMS coating can be as simple as a mixture of

silanol (SiOH) functional PDMS, fillers, and an endblocker

or crosslinker for the silanols which is also reactive with

moisture present in the atmosphere. Catalysts can be added

to accelerate the moisture activated cure.

There are several types of cure systems available. The

1-part and 2-part condensation cure systems result in siloxane (Si-0-Si) bonds and release a volatile leaving group. The

2-part addition cure results in an ethylene (Si-CH2-CH2-Si)

bond with no leaving group.

1-part Moisture Activated Cure

Basic 1-Component Formulation

Silanol functional PDMS 100 parts

Fumed Silica 6 - 15 parts

(treated or untreated)

Crosslinker 3 - 7 parts

 (acetoxy, oxime, methoxy)

Catalyst 0.1-0.5 parts

Polymer

Linear PDMS terminated with silanol groups is the

starting polymer. Two aspects must be balanced in order to

select the best polymer. First, the modulus and toughness of

the coating will partly depend on the polymer molecular

weight and resulting crosslink density. Second, the coating

viscosity will also be effected by choice of the polymer. The

two factors should be balanced to provide a coating that will

provide good application properties at a solids level that will

meet Volatile Organic Content (VOC) requirements. Such

polymers have a molecular weight (Mn) range of

20,000-40,000.

Filler

Fumed silica is incorporated into the PDMS polymer

under high shear. This type of high surface area silica
Four/1997 43

(150-300 m2/gm) is used in almost all PDMS coatings and

sealants to provide strength. The surface silanol groups interact with the siloxane bonds through hydrogen bonding

and create a “mechanical” reinforcement. They also react

with the crosslinkers forming a covalent bond. The tensile

strength of a non-filled, crosslinked PDMS is about 20 psi.

The same polymer/crosslinker mixture with 15% highly functional silica filler could have a tensile strength of more than

600 psi or a 30-fold increase (31). However, high levels of

these high surface area reinforcing fillers will result in higher

viscosity formulations.

The treatment of the silica surface with Me3Si0- or

Me2SiO- results in a hydrophobic surface. These treated

silica’s are more easily dispersed in PDMS and have a lower

moisture content, but provide lower reinforcement. High

surface area calcium carbonates are often used in sealants to

provide higher tear strengths. Other pigments can also be

used to provide opacity, color or harder coatings. Titanium

Dioxide, iron oxides, carbon black, and a range of other

materials are typical for silicone elastomeric sealants and

coatings.

Crosslinker/Catalyst

The moisture reactive crosslinker is added under anhydrous conditions at a concentration where all of the silanols

on the PDMS are endblocked. A deficiency of crosslinker

will result in a viscosity increase or even gelation. Typical

crosslinkers are methyltriacetoxysilane,

methyltris-(methylethylketoximo) silane and

methyltrimethoxysilane. Typical catalysts are metal carboxylates, alkylmetal carboxylates and alkylmetal alkoxides such

as dibutyltindiacetate, dibutyltindilaurate, tetrabutyltitanate

and tetraisopropyltitanate.

Aliphatic solvents such as mineral spirits can be used

to reduce the coating to a suitable application viscosity. Several other solvents may also be used, such as

methylamylketone, isoparaffinic solvents such as Isopar G,

and recently use of volatile methyl siloxanes (VMS) with

are low molecular weight linear or cyclic dimethylsiloxanes.

Curing

Cure is accomplished by the reaction of the crosslinkers

with moisture in the air forming the stable siloxane (Si-O-Si)

bond and emitting the reactive group as acetic acid,

methylethylketoxime or methanol. Cure starts at the surface

and by proper selection of the crosslinker and catalyst, tack

free times of less than 30 minutes can be achieved. The cure

rate is effected by both the relative humidity and the temperature. For example, a coating that will be tack free in 30

minutes at 25 C and 50% relative humidity may take longer

than 60 minutes at 20 C and 30% relative humidity and only

15 minutes at 30 C and 90% relative humidity.

Two-component Coatings

Moisture Cure

Two component coatings consist of a Part A containing the silanol functional PDMS, fumed silica, other pigments and solvent and a Part B which contains either

ethylorthosilicate or propylorthosilicate sufficient to crosslink

the silanols. The catalysts normally used are stannous octoate

or dibutyltindilaurate. Upon mixing, the alkoxide sites react

with the silanols to form the Si-O-Si bond and the corresponding alcohol.

Two-part Addition Cure

In two-part addition cure systems, Part A usually consists of a vinyl functional PDMS with reinforcing fillers.

Sometimes a vinyl functional methylsilicone resin is used in

place of the silica. All of the reinforcements are treated so

that the SiOH and water content are minimal. This prevents

reaction with the crosslinker which creates hydrogen gas.

Part B contains SiH functional silane or siloxane which

when catalyzed reacts with the vinyl sites to create the ethylene crosslink. Typically, chloroplatinic acid (H2PtCl6) is

used as the catalyst.

Cure System Comparison

The following table summarizes some of the general

advantages and disadvantages of different possible cure systems. Although many other types of cure systems are known,

those listed are most commercially utilized. Understanding

the trade-offs which are made can assist with choice of the

Table 1

Comparison of widely used commercial silicone elastomer

cure systems.
44 Naval Research Reviews

best possible system for a particular application, such as a

fouling release coating.

Future of Silicone

Fouling Release

Coatings

Due to environmental regulations on toxic antifouling

materials, a non-toxic solution for control of fouling is desired. Silicone elastomeric coatings based on

polydimethylsiloxane (PDMS) have been identified as the

best current alternative for fouling control. Silicone technology for release applications is broadly known and dominates several industries including paper coatings. The unique

structure of PDMS imparts a low surface energy coating due

to tightly packed methyl groups, and a low Tg material which

provides a weak diffuse interphase. Although the overall

mechanism for release of marine biofouling organisms is not

known, it likely involves the structure and properties mentioned.

Many types of formulations can be prepared, including variations in polymer molecular weight, filler type

and level, crosslinker and catalyst type and level, and solvent type and level. The specific formulation used will depend on the relative fouling control needed, the toughness

necessary, and the application properties needed.

Currently, polydimethylsiloxane fouling release coatings are being evaluated for application in cooling water inlets of power plants and other stationary structures where

they have been shown to be effective against marine fouling

including the fresh water Zebra Mussels. In addition, practical evaluation in the form of ship test patches is being carried out by the US Navy. The purpose of this testing is to

determine real world performance for control of fouling and

durability of the systems, especially when cleaned via underwater scrubbing methods, and thus define whether these

materials are operationally viable.

Although it is clear that the current non-toxic fouling

release coatings are not as efficacious as ablative tin coatings, they do represent an environmentally friendly alternative to dealing with the problems of fouling. To obtain better

performance in the short term, it may be necessary to utilize

coatings with non-toxic fluid additives, or perhaps with low

release rates of biodegradable organic antifouling agents.

Additional development needs to be completed to identify

commercially viable means of combining the antifouling

agents and PDMS based coatings. Also, in the next several

years, testing should continue on ideas for improving the

overall toughness and fouling release properties of the PDMS

coatings. Continued research to identify the fundamental

mechanism for improving release should also be pursued, in

order to guide future development efforts aimed at providing self cleaning vessels.

Biographies

Gregory G. Bausch received a B.S. Degree in Chemistry/Mathematics from the University of WisconsinLaCrosse in 1987. Mr. Bausch has worked at Dow Corning

since graduation, and has led development efforts for several silicone fluids, resins, and emulsions. He is currently a

Group Leader/and Development Specialist for Coatings and

Additives. Mr. Bausch has 5 patents and numerous 3 publications.

Harold L. Vincent received his B.S. Degree from

Michigan State University in 1954. His career in the silicone industry spans 40 years of research, development, and

commercialization of silicone resins, composites and coatings at the Dow Corning Corporation. Mr. Vincent retired

as an Associate Scientist in 1993 and is now active as a consultant on a variety of silicone technologies including fouling release coatings. He has published numerous papers,

holds 25 patents, and has made technical presentations

throughout the world.

REFERENCES

1. Kjaer E.B., Bioactive Materials for Antifouling

Coatings, Progress in Organic Coatings, 20 (1992)

339-352.

2. Organotin Antifouling Paint Control Act of 1988

3. Sisson W., Bottom Paints: Getting Greener, Soundings (March 1994) A32.

4. Thorn F., Water Quality Standards, National Paint

and Coatings Marine and Offshore Conference, 1993,

5. Muller, W., and Noacki, L., US3702778, 1972.

6. Kroyer K., GB1307001, 1973.

7. Milne A., US4025693, 1977.

8. Milne, A., and Callow, M., Non-Biocidal Antifouling

Processes, Trans IMarE (C) Vol 97, Conf. 2, Paper

37, 1984.

9. Callow, M., Pitchers, R., and Milne, A., The Control

of Fouling by Non-Biocidal Systems, Stud. Environmental Sci. 28 Algal Biofouling, 145-158, 1986.

10. Chugoku (Assignee) J58013673, 1983.

11. Ibid, GB2141436, 1984.

12. Kishihara, M., Nanishi, K., and Yonehara, Y.,

US5218059, 1993.

13. McLoughlin, B., et. al., GB1581727, 1980.

14. Shone, E., and Riches, K., GB2096019, 1981.

15. Riches, K., et. al., GB7942682, 1981.

16. Fox, H., Taylor, P., and Zisman, W.,

Polyorganosiloxanes: Surface Active Properties,

Industrial and Eng. Chem., Sep. 1947, 1401-1409).

17. Owen, M.J., Low Surface Energy Inorganic Polymers, Comments, Inog. Chem., Vol. 7, No. 4, 1988,

195-213.

18. Owen, M.J., Encyclopedia of Polymer Science and

Engineering, 1988, Volume 14, pg. 411- 421.
Four/1997 45

19. Jabbari, E., and Peppas, N.A., Polymer-Polymer

Interdiffusion and Adhesion, J.M.S-Rev. Macromol.

Chem. Phys., C34(2), 1994, pgs.205-241.

20. Goupil, W., DePalma, V., and Baier, R., Prospects for

Nontoxic Fouling Resistant Paints, Proceedings 9th

Annual Conference on the Marine Technology

Society, Washington, D.C., 1973.

21. Goupil, W., DePalma, V., and Baier, R., Physical/

Chemical Characteristics of the Macromolecular

Conditioning Film in Biological Fouling, Bio. Mar.

Comun. Congr. Corros. Mar. Incrustaciones, 5th.

401-410, Ed. Garsi, Madrid, Spain.

22. Dexter, Stephen C., Influence of Substratum Critical

Surface Tension on Bacterial Adhesion; Journal of

Colloid and Interface Science, Vol. 70, No. 2, June

15, 1979.

23. Schrader, Malcolm E., On Adhesion of Biological

Substances to Low Energy Solid Surfaces; Journal Of

Colloid and Interface Science, Vol. 88, No. 1, July,

1982.

24. Baier, R., Why Zisman/NRL Critical Surface Tension

Better Predicts Biological Fouling Resistance Than

Do Undewater Contact Angles, ONR Seminar, Oct.

1994.

25. Murase, H., and Nanishi, K., Studies on the Adhesion

Control of Marine Organisms by Means of a Heterogeneous Polymer Surface, Kobunshi Ronbunshu,

50(11), 837-45, 1993.

26. Kishihara, M., Nanishi, K., and Yonehara, Y.,

US5218059, 1993.

27. Chaudury, M.K., Lin, R., and Owen, M.J., Dow

Corning Internal Report, 1987-I0030-6182 (1987).

28. Owen, M.J., Chemtech, 11, 288 (1981).

29. Owen, M.J., Gentle, T.M., Orbeck, T., and Williams,

D.E. in “Polymer Surface Dynamics”, (J.D. Andrade,

Ed.), Plenum Press, NY, p. 101 (1988).

30. Schmidt, R.G., Vincent, H.L., Swain, G., Polymer

Surface Tension Dynamics in Sea Water; Influence

on Marine Antifouling Performance, Unpublished

Work.

31. Klosowski, J., Sealants in Construction, Marcel

Dekker, Inc.,1989.
46 Naval Research Reviews

Introduction

A key component to antifouling research is a well structured test and evaluation program. This must be tailored to

provide scientific information to research groups and operational performance to the end users. The development of

non-toxic antifouling coatings has provided some unique

challenges with regard to selecting and developing test protocol. During the 1994 Office of Naval Research (ONR)

Biofouling Contractors Workshop, a list of criteria considered important for the evaluation and understanding of such

coatings was developed (Figure 1). These were categorized

under four main headings: operational requirements, physical properties, chemical properties, and biofouling properties.

The operational requirements of an antifouling system are fairly well understood and reflect the needs of the

US Navy to preserve the submerged portion of ship and submarine hulls in a smooth and foul free condition. The long

term objective is to develop a coating which provides equal

or better performance than the self polishing organotin coatings, at similar cost, but with out the adverse environmental

impacts. Many of the chemical and physical properties can

be found through standard test protocol1

, however, new techniques and instrumentation are being developed and these

enable a more detailed understanding of coating characteristics. The biofouling properties are determined using both

laboratory and field techniques. This paper focuses on

biofouling tests, and in particular new methodologies that

have been developed for panel testing of non-toxic surfaces

under static immersion conditions.

Biofouling Evaluation

Over the years, a variety of techniques have been developed to assess the effectiveness of both natural and synthetic compounds as antifouling agents. These, traditionally,

have been directed to evaluate biocidal compounds, however, the mode of action of non-toxic coatings is quite different (Figure 2). Marine organisms that settle on a toxic

Field Evaluations of

Non-Toxic Antifouling

Coatings: New Field

Technologies and

Performance Criteria

Geoffrey W. Swain, Florida Institute of Technology, Melbourne, FL

“It is the mark of an instructed mind to rest satisfied with that degree of

 precision which the nature of the subject admits, and not to seek

exactness where only an approximation of the truth is possible” --Aristotle.
Four/1997 47

surface may survive short periods, but providing the coating

is functioning, they soon die or become detached. The coating remains free of fouling. A non-toxic surface, however,

may become totally covered by fouling organisms, and its

effectiveness then becomes a function of the ease with which

the biofouling can be removed.

With-in the ONR biofouling program, a hierarchy of

tests has been established to streamline the evaluation of formulations that are being developed. A coating or compound

is first screened using short term laboratory tests. These include the use of bioluminescent bacteria2

, and barnacle in

vitro assays using Balanus amphitrite3. The advantage of

such tests is that they only require small quantities of the test

material and this eliminates the need for large volume production of a non-proven prototype system. If the formulation shows promise, it is then subjected to rod testing4

.

The rod test is a field evaluation that was designed to

bridge the gap between laboratory findings and long term

panel tests. Candidate coatings are applied to fiberglass rods,

110 mm long and 7 mm diameter. These are mounted, in

binary comparison, at 7.5 cm intervals in 50 cm by 50 cm

square racks made of 15 mm internal diameter PVC pipe

supporting Vexar 10 mm (long dimension) diamond mesh

screening. For a test, 20 rods of each coating type are deployed in a blocked and randomized array. The racks are

deployed in the marine environment and the rods monitored

for the first signs of macrofouling. When fouling is detected,

5 rods of each type are removed and the fouling counted

each week for one month. Uncoated fiberglass rods and AF

121 cuprous oxide coated rods provide a reference for the

effectiveness of new formulations. Promising formulations

from the rod testing are then applied to panels for long term

static immersion.

Long Term Panel Testing

Panel testing provides the first opportunity to subject

coatings to a broad spectrum of physical, chemical and biological action over a prolonged time period (Table 1). Candidate coatings are applied to standard 254 x 305 mm (10 in.

x 12 in.) test panels with 9.5 mm (3/8 in.) holes at each corner to facilitate attachment to the test frames. These are immersed at a static exposure site with known fouling

communities.

The selection and design of methods to evaluate the

performance of the test panels was made on the basis of the

following questions:

· What information will be useful to improve the

knowledge of the research scientist?

· What information will be useful to enable the Navy

to select the best system for their needs?

· How can the test results be related to performance?

· What are the limitations of the type of test selected?

· How should the precision of the work be adjusted

to be in accord with the limitations of the method and yet

achieve economy of effort and consistent reliable results?

· How many samples are necessary to obtain representative results?

The most obvious parameters to monitor are the rates

Figure 1

Criteria used to evaluate the performance of antifouling

coatings.

Figure 2

Settlement matrix for toxic and non-toxic surfaces.
48 Naval Research Reviews

of biofouling accumulation and the physical deterioration.

A test procedure for antifouling coatings containing active

biocides was already available (ASTM D 3623 - 78a (1987)

“Standard method for testing antifouling panels in shallow

submergence”5) and this was adopted for the general management and observations of the test panel performance. The

non-toxic systems, however, may become fully fouled and it

is then the strength of biofilm adhesion to the substrate that

determines the effectiveness of the coating. To quantify

biofouling adhesion we designed two test methods:

· Water jet evaluation

· Barnacle adhesion measurements.

Both methods were designed to be fully portable, field

friendly and economic with regard to effort and time.

It must be remembered that biofouling is a dynamic

process in which community structure is for ever changing.

The interpretation of results must take into account seasonal

and spatial differences at individual test sites, and community differences on a global scale. In addition, the coating

itself will age. It is our experience that for short term exposure (less than 12 months) of panels that do show some antifouling characteristics, only the results from panels that were

placed under static immersion at the same time should be

directly compared one to another. It is also important to test

replicate panels at several sites world wide and that these

should be calibrated to coatings of known performance. This

means that forward planning of test panel exposure is of the

utmost importance to get meaningful and comparative information.

Test Protocol

The following methods have been established for the

test and evaluation of non-toxic antifouling coatings:

· Biofouling and physical condition of the coating.

· Biofouling adhesion measurements using a calibrated water jet.

· Barnacle adhesion measurements in shear.

The standard calibration surfaces used in the present

Navy Program are F150 Epoxy, BRA Copper Ablative, Dow

Corning RTV 3140, General Electric RTV 11 and International Intersleek.

Biofouling and Physical

Condition of the Coating

The biofouling and physical condition of the coating

are evaluated according to ASTM D 3623 5. This standard

was developed as a method to evaluate antifouling coatings

that contain biocides as their mode of action. It describes

how to organize a static immersion site, how to maintain a

biofouling attachment census and details methods to evaluate the surface for fouling and physical condition.

Fouling is rated according to the percentage of the intact proportion of coating or surface that is covered by a particular type of fouling organism as classified in Table 2.

Fouling on portions of a panel where no test surface is present

(due to coating failure, e.g. delamination) is ignored. The

following definitions are made with regard to the reporting

of fouling and coating condition.

· Absorbed organic and inorganic chemicals, trapped

silt and detritus, should be reported as “silt”.

· Diatoms, initial algal germination and low form algae should be reported as “algal slime”.

Table 1

Variables associated with field testing antifouling coatings.

Table 2

Description of fouling types according to ASTM D 36232

.
Four/1997 49

· Immature foulers should be reported as “incipient

foulers”.

· Mature foulers should be reported as number of individuals present or as percent cover for colonial forms.

· The physical condition of the surface should be rated

according to percentage of surface affected by coating defects (Table 3), and the type of coating deterioration should

be described qualitatively.

This method then awards a rating for Fouling Resistance (F.R.) and Physical Rating (P.R.). For coating surfaces

free of fouling except for the presence of algal spores and

other biological slimes, a F.R. rating of 100 is given. This is

reduced to 95 if only incipient fouling is present, and when

mature fouling is present the rating is further reduced by

subtracting the percentage cover or the sum of the number

of individuals present from 100 or 95. The P.R. is obtained

by subtracting the percent surface affected by defects from

100. The overall performance is obtained by awarding the

coating the lowest rating from the fouling resistance and

physical condition.

Biofouling Adhesion Measurements Using a Calibrated Water

Jet

The water jet test was developed to provide an assessment of the adhesion strength of silts, slimes, and low form

fouling types to surfaces where biofouling adhesion is low.

It was designed to be fully portable and self contained for

field operations and this limits the maximum water jet force

that can be applied.

The apparatus consists of a SCUBA tank containing

compressed air and a regulator to adjust the pressure between

0 - 1.65 MPa (0 - 240 psi), a SCUBA tank containing water,

and a quick change blow gun with a 1.6 mm (1/16 inch)

diameter nozzle to apply the water jet (Figure 3). A 50 x 50

mm template is used to delineate the surface to be evaluated,

and the test procedure is fully recorded by a video camera.

The measurements are made on an area that is defined

by a 50 x 50 mm template placed over a representative part

of the surface. The biofouling communities of this area are

assessed and recorded using a video camera. The regulator

is set to 0.138 MPa (20 psi) to pressurize the SCUBA tank

containing water. The resulting water jet is applied to the

area within the template in a manner so as to maximize fouling removal. The operator accomplishes this by changing

the angle and distance of the water jet to the surface until the

maximum amount of fouling that can be removed at that

setting is achieved. The biofouling cover of the surface is

reassessed and recorded by the video camera. The pressure

in the system is increased by 0.138 MPa (20 psi) and the

cycle repeated until all the fouling is removed or the maximum pressure setting is obtained.

For evaluation purposes, the biofilm is divided into

silt, slime, soft and hard foulers. The waterjet pressure required to remove each of these groups is recorded. The maximum true force exerted by the water jet at the surface for

each pressure setting was measured using a force transducer

and by observing the area over which the water exerted an

effect.

Barnacle Adhesion

Measurements in Shear

The barnacle adhesion test was designed to enable rapid

and multiple measurements of barnacle adhesion strength in

shear to a coating under field conditions. It was approved as

an ASTM standard in 1994 6, and it provides quantitative

data that can be used to compare the ability of surfaces to

reduce biofouling adhesion. The method was based on previous research conducted on the adhesion strengths of barnacles to natural substrates 7,8,9,10, 11.

Test surfaces that are exposed to static immersion are

monitored for barnacle settlement. To facilitate adult barnacle growth on silicones with low fouling adhesion

strengths, it has been found necessary to enclose the test area

with a 25mm (1 inch) mesh net that excludes larger fish and

other predators which would otherwise remove the immature forms. When settlement is observed, the shear strength

of adhesion is measured as described below. Typically live

barnacles between 5-20 mm diameter at the base are selected

for testing, and their species and condition are noted. They

should be at least 20 mm from the edges of the test panel, be

attached to undamaged areas of the test surface, and not be

in direct contact with other barnacles. The barnacle base is

measured with calipers in four directions (0, 45, 90, 135 degrees) to obtain an average base diameter . The barnacle

base plate area, A, is then estimated using the average base

Figure 3

Water jet apparatus.
50 Naval Research Reviews

diameter, da and the formula A = (πda

2

)/4. A shear force is

then applied to the barnacle base using a hand held force

measuring device at a rate of approximately 4.5N/s (1.0 lb/

s) until the barnacle becomes detached or the maximum shear

force is reached. The force measuring device used in the

this experiment was a Shimpo mechanical dial force gauge.

This device had a range of 0-89 N (0-20 lb), an accuracy of

±0.3%, and a resolution of 0.45 N (0.1 lb). Care must be

taken to apply the force parallel to the surface. The force (F)

required for detachment is noted, and observations are made

as to the mode of failure. If more than 10% of the barnacle

base plate is left attached to the substrate, then the test is

deemed void. The strength of adhesion, τ pascals (psi), is

then calculated by dividing the force required to remove the

barnacle, F newtons (pounds), by the area of the barnacle

base plate, A square meters (square inches).

τ = F/A

It is recommended that, when possible, a minimum of

ten measurements be made per coating, and the mean and

standard deviation of the strength of adhesion, τ, are calculated. The strength of adhesion values for test surfaces are

compared to control surfaces of known adhesion. Variations

in barnacle adhesion strength may result from differences in

species, age, time of year, and physical condition. Additionally, incomplete removal of the barnacle base plate from the

surface during testing may lead to erroneous adhesion values. Care must also be taken not to dislodge the barnacle

during base diameter measurements.

Summary

The use of the static immersion panel evaluation methods provides some new and interesting insights into the performance of non-toxic coatings. It is now possible for

researchers to quantify how modifications to their formulations alter the strength of biofouling adhesion, and for the

Navy to select the coatings with the best foul release properties. Research is currently underway to relate biofouling

adhesion strength to mechanical cleaning methods and to

the ship speed required for self cleaning. This knowledge

should help in the development of non-toxic antifouling coatings and ship hull maintenance practice that will remove the

necessity for the use of biocides.

Acknowledgments

This work was supported by the Office of Naval Research (Grant No N00014-91-J-1465). I would like to thank

Dr. Randall Alberte and Captain Steven Snyder for their support of this program, Mike Schultz for helping to develop

the test methods, and all the participants of the ONR

Biofouling Program for their lively interest and input.

Biography

Dr. Geoffrey Swain is an Associate Professor in the

Ocean Engineering Program at the Florida Institute of Technology. He has an active research group specializing in corrosion and biofouling control in marine environments.

REFERENCES

1. American Society of Testing and Materials, Paint -

Tests for Formulated Products and Applied Coatings.

Volume 06.01, 1996.

2. Angell, P., Arrage, A., White, D.C. “On-line, nondestructive monitoring of microfouling on painted

antifouling surfaces”. Non-Polluting Biofouling

Workshop, Monterey, CA Nov. 1992.

3. Rittschof, D., Clare, A.S., Gerhart, D.J., Sister Avelin

Mary, Bonaventure, J. “Barnacle in vitro assays for

biologically active substances using mass cultured

Balanus amphtrite amphitrite Darwin”. Biofouling V.

6, No. 2, 1992.

4. Rittschof, D., Clare, A.S., Gerhart, D.J.,

Bonaventure, J., Hadfield, M.G. “Rapid field

assessment of antifouling and foul-release coatings”.

Biofouling V. 6, No. 2, 1992.

5. ASTM D 3623 - 78a (1987) “Standard method for

testing antifouling panels in shallow submergence”,

American Standard for Testing and Materials.

6. ASTM D5618 (1994) “Standard test method for

measurement of barnacle adhesion strength in shear”,

American Standard for Testing and Materials.

7. Becka A, Loeb G (1984) “Ease of removal of

barnacles from various polymeric materials”,

Biotechnology and Bioengineering, XXVI, 1245-

1251.

8. Bultman J D, Griffith J R, Field D E (1981)

“Fluoropolymer coatings for the marine environment”, In Proc. Symp. on Marine Biodeterioration,

eds. J.D. Costlow and R.C. Tipper, pp 237-249,

Naval Institute Press, Annapolis, MD.

9. Denny M W (1988) Adhesion. In Biology and the

mechanics of the wave-swept environment. Princeton

University Press, pp. 228-237.

10. Grenon J F, Elias J, Moorcroft J, Crisp D J (1979) “A

new apparatus for force measurement in marine

bioadhesion”, Marine Biology, 53, 381-388.

11. Swain G W, Griffith J R, Bultman J D, Vincent H L

(1992) “The use of barnacle adhesion measurements

for the field evaluation of non-toxic foul release

surfaces”, Biofouling, V. 6, No. 2, 105-114.
Four/1997 51

Introduction

The search for effective means to control biofouling

accumulation on manmade surfaces in aquatic environments

began with the first attempts to explore and exploit the sea.

Today, the need for materials that prevent biofouling in

aquatic environments is approaching a crisis. Since the

1970’s organometallic compounds such as tributyl-tin (TBT)

have become the industry standard because they produce

economical, highly effective and long-lasting antifouling

(AF) coatings. Unfortunately, these highly effective materials are also extremely toxic and environmentally recalcitrant,

which has led to serious problems of environmental pollution associated with their application, use and disposal. The

gradual abandonment of organo-tin based antifoulants has

resulted in a significant increase in the use of less effective

copper-based coatings in the last 10 years. Not surprisingly,

the increased use of copper-based AF coatings has been

blamed for recent but dramatic increases in levels of copper

found in marine sediments1

 and in mussels (Mytilus

californianus) used as indicators of marine pollution15, suggesting that copper-based coatings may also present unacceptable environmental hazards. In Europe, the triazine

herbicide Iragol 1051 has been added to Cu-based AF coatings for more effectiveness in the wake of the ban on TBT.

This compound accumulates in fish and decomposes very

slowly in seawater, raising concerns about the potential environmental hazard associated with its use in AF coatings10.

Nature has evolved a variety of strategies that prevent

or minimize fouling of natural surfaces in aquatic environments without resulting in permanent environmental degradation or accumulation of refractory and toxic substances.

Some organisms use physical means, including the sloughing of the outer tissue layer4,9,13 the generation of an external

surface that minimizes bioadhesion (Vroilick et al. 1990),

and the production of secondary metabolites that can deter

potential fouling organisms14 and predators6. The chemical

species employed as defensive agents are as diverse as the

Selection, Design and

Delivery of

Environmentally

Benign Antifouling

Agents

Donald C. Sundberg and Niraj Vasishtha, Polymer Research Group, Dept of Chemical Engineering, University of

New Hampshire, Durham, NH

Richard C. Zimmerman, Dept of Biology, University of California at Los Angeles, CA

Celia M. Smith, Dept of Botany, University of Hawaii at Manoa, Honolulu HI
52 Naval Research Reviews

organisms manufacturing them. Those known agents include

elemental vanadium, inorganic acids, and a variety of organic compounds including saponins, terpenoids and phenolic acids3

. An important goal of the non-toxic antifouling

research program supported by the U.S. Navy, Office of

Naval Research (ONR) has been to understand and exploit

natural agents and their analogs that promise effective

biofouling control without polluting the environment.

New Antifouling Agents

U.S. patents for natural antifouling agents were first

awarded in the 1880’s for the use of relatively crude tannic

substances extracted from terrestrial and aquatic plants14. The

relatively low effectiveness of these tannins, however, has

precluded their practical use in AF coatings. In recent years,

sophisticated chemical techniques have been used to identify a number of specific secondary metabolites from invertebrates and plants that show promising AF activity11,16,17,18,22.

In order to be considered for the next generation of nonpolluting AF coatings for use by the U.S. Navy, candidate

agents must be:

(i) effective at concentrations that do not present a toxic

hazard to non-target organisms,

(ii) short-lived in the environment and decompose to

prevent food-web bio-accumulation or accumulation to toxic

levels through sediment deposition,

(iii) economical to produce without exploiting or endangering natural populations,

(iv) safe to apply and remove in coatings, and

(v) compliant with federal regulations for minimizing

air/water pollution (EPA) and worker exposure (OSHA). In

essence, our objective is to exploit natural methods of

biofouling control to produce effective and environmentallysafe protection of manmade structures.

The natural agents currently under investigation include terpenoids and a sulfated phenolic acid (Fig. 1). The

terpenoids renillafoulin and pukalide were isolated from

marine coelenterates11,16, while juncellin was isolated from a

terrestrial angiosperm native to tropical India (Sister Mary

Avelin pers comm). The sulfated phenolic acid (zosteric acid)

was isolated from the marine angiosperm Zostera marina

(eelgrass18).

Synthetic routes for generating these compounds and/

or their derivatives were explored because their isolation from

organisms represents prohibitive production costs and unacceptable impacts on natural populations. It was possible to

synthesize zosteric acid faithfully from readily available precursors, but structural complexity prevented de novo synthesis of the terpenoids in the laboratory. Consequently,

smaller chemical analogs were employed that reproduced

enough of the original chemical structure to maintain biological activity (Fig. 1). The chlorinated isothiazolone, SeaNine 211 (Rohm & Haas Co.), was also evaluated in this

program. Unlike Iragol 1051, Sea-Nine 211 appears to have

a very short half-life in seawater, reducing the potential for

it to accumulate to toxic levels in the environment.

Initial screening of AF effectiveness of individual compounds involved static laboratory assays in which agents were

dissolved or dispersed in seawater containing marine bacteria, diatoms or invertebrate larvae and a suitable substrate

for attachment, such as a glass slide. The assays generally

produce a concentration-dependent attachment response from

the target organism that can be used as an initial indicator of

AF effectiveness (Fig. 2), but the static nature of these tests

can make it difficult to extrapolate the potential effectiveness to real-world conditions.

Mode-of-Action Studies

In addition to being classified as natural agents, natural analogs of natural agents or synthetic agents, candidate

AF compounds can also be categorized by their mode-ofaction in preventing biofouling. Potential AF mechanisms

involve (i) acute toxicity, (ii) avoidance or repellence and

(iii) non-toxic inhibition of attachment.

Figure 1

Candidate antifouling agents and their structures, sources

of these products and their modes-of-action.
Four/1997 53

Acute toxicity is the highly effective mode-of-action

of organo-tins, copper and the synthetic biocides (Iragol 1051

and Sea-Nine 211). These compounds do not prevent attachment, but chronic exposure is fatal to the developing organisms after attachment. Unfortunately, the indiscriminate

nature of this mechanism can affect non-target organisms as

well. Repellence involves chemosensation and avoidance

behavior, causing organisms to move away from the protected surface. Although repellents may be toxic in high

concentration, the elicitation of escape behaviors at low concentrations suggests that repellents can be engineered to be

effective close to the protected surface at concentrations that

do not threaten non-target organisms. A repellent mode-ofaction will not, however, be effective against unresponsive

organisms.

A novel approach to biofouling control that can prevent attachment through non-toxic means involves compounds that bind to cell membranes or cell walls/extracellular

matrices, thereby altering cell surface properties and affecting the ability of organisms to attach to surfaces. Zosteric

acid appears to fall into this category of antifouling agents.

The specific mechanism can involve changes in cell surface

hydrophobicity that prevent cells such as bacteria from interacting with filmed surfaces that may themselves possess

cues for settlement8

, interaction with specific receptor molecules (e.g. lectin binding sites) on the surfaces of bacteria

and algal spores that prevent binding to molecules present

on the target surfaces (Fig. 3), or blocking the polymerization or “curing” of proteinaceous adhesives secreted by invertebrates.

The search for new AF agents that are broadly effective at low flux rates and even more benign to the marine

environment continues. An important breakthrough for the

Pacific Test site and our understanding of fouling by tropical organisms was the development of a settlement bioassay

with larvae of Hydroides elegans, a polychaete tubeworm

and the principal fouler of Pearl Harbor. This single advance substantially accelerated ONR testing from one month

in the field to one week in the laboratory5

 with full statistical

rigor. Our strategy has been two-fold:

(i) focus on those tropical organisms that appear to

have solved the biofouling problem without shedding their

surfaces4 and (ii) assay their surfaces as well as possible compounds that they may release into surrounding waters. Using H. elegans larvae as a biological probe, we have

identified other marine organisms that can (i) inhibit settlement, (ii) increase settlement or (iii) result in the same extents of settlement by H. elegans as if the larvae were exposed

to only filtered seawater. In all trials, we discriminate those

compounds that clearly deter settlement (solid bars Fig. 4)

from concurrent experiments for settlement under control

conditions (patterned bars, with nested ANOVA,

* = p < 0.05). Findings of this study21 (Fig. 4) indicate that

settlement can be delayed without longer term effects on viFigure 2

Dose-response curve for the ability of zosteric acid to inhibit

settlement by the calcareous polychaete Hydroides elegans

in static laboratory experiments, plotted as a percentage of

larvae set in control dishes that contained no zosteric acid.

Error bars represent 1 sd of the mean.

Figure 3

Compounds that inhibit attachment such as zosteric acid

interact with surface moieties on target organisms to prevent

adhesion of cells to the protected surface. Such inhibitors

are non-toxic and dissociate from the organisms at low

concentration.
54 Naval Research Reviews

ability of larvae; future research into the identity of these

compounds will likely be fruitful.

Delivery of AF Agents

From Coatings

To be effective, an AF agent must be delivered to the

coating surface at a desired rate. This release rate, or flux, is

traditionally expressed in units of micrograms per square

centimeter of surface area per day (μg cm-2 d-1 ). Depending

upon the type of coating being evaluated, this release rate

may or may not be constant. It goes without saying that

constant release rates are preferable in that they yield efficient use of the AF agent throughout the lifetime of the coating. Figure 5 displays the relationship between the expected

useful life of a constant release rate coating and the flux necessary to be effective against fouling. The two curves in the

figure represent the results for coatings of different thicknesses, both containing 25 wt % AF agent at the time of

application. Here it is clear that in order for a coating to last

for 5 years, the release rate required for effective control of

biofouling cannot be larger than 5 to 10 μg cm-2d-1. This provides a useful basis upon which to select among candidate

AF agents. Unfortunately, developing a coating which yields

a constant release rate over a sustained period of time (5 to 7

y for Naval applications) is not a trivial task.

There are several distinctively different types of marine coatings that need to be considered because coatings

typically fall in two categories, each with unique characteristics that influence release rate properties. The first type of

coating has polymers that constitute the matrix phase of the

coating and can be made to dissolve slowly, or ablate, into

sea water. The dissolution rate of the matrix phase controls

the rate at which AF agents are delivered to the marine environment. A second, non-ablative type of coating, also contains AF agents but relies on diffusion processes within the

coating to deliver an AF agent to the marine interface. These

non-ablative coatings can be porous or non-porous. For porous coatings, sea water penetrates somewhat into the coating to dissolve an AF agent and to provide a pore through

which the agent is delivered to the surface. For non-porous

coatings, molecular diffusion through the polymer results in

delivery of the AF agent to the surface. Because future coatings are likely to be non-porous and perhaps non-ablative,

we will discuss these in somewhat greater detail.

Monolithic Coatings

When an AF agent is freely dispersed in a polymer

binder, the coating is described as monolithic. Molecular

diffusion is responsible for the transport of the AF agent

through the coating and diffusional characteristics will depend on the concentration of the agent relative to its saturation limit in a particular polymer. Mathematical descriptions

of the release rate can be obtained and such expressions show

that release rates depend on the diffusion coefficient in the

polymer and the concentration of the agent. Concentration

gradients develop within the coating during the release period and the diffusion coefficient will often be dependent

upon the local concentration. This results in an often complex quantitative expression for the release rate, but analyses are available. Higuchi7 has described the flux through

coatings in which the agent is initially present above its saturation limit. For unsaturated conditions the analysis is somewhat simpler and diffusional analyses are available2

.

Figure 4

Inhibition of settlement by Hydroides elegans when exposed

to seawater conditioned by two marine algae, Sargassum

echinocarpum and Sphacelaria tribuloides. Within each pair

of bars, solid bars represent the percentage of larvae that

settled within one day of exposure to treated water (T);

patterned bars represent the percentage of larvae that settled

within one day under control conditions (C). (* = p<0.05 nested

ANOVA21).

Figure 5

Release rate, or flux, necessary to prevent biofouling as a

function of the expected coating life when the AF agent is

released at a constant rate for coating thicknesses of 100

and 500 μm. Coatings initially contained 25 wt% AF agent,

independent of coating thickness.
Four/1997 55

For the simpler case where the concentration of an AF

agent is below its saturation limits in a particular polymer,

the release rate will always be variable because the concentration of the AF agent decreases with time. Evaluating the

time dependent release rates from such simple coatings allows us to characterize a diffusion coefficient of an agent in

the polymer. Here we use an approximate form of the flux

equation from Crank2

 as shown in equation 1.

Mt/ M() = 4{π}1/2[Dt / a2 ]1/2 (1)

where Mt is the total amount of the agent that has diffused

out of the coating at time t, M() is the amount of agent in the

coating, D is the diffusion coefficient, and a is the radius of

the cylinder (we have taken the case of a cylindrical coating). The amount released per unit area of the coating, Q, is

related to Mt 

 as :

Q = Mt / {2 π a L} (2)

where L is the height of the cylinder. With data for the value

of Q as a function of time, we can characterize the release

rate profile for the coating and determine the apparent diffusion coefficient by evaluating the slope of a Q versus tl/2

plot. Figures 6 and 7 show some data for the release of SeaNine 211 (isothiazolone C11H17NSOCl2

). From such data,

the diffusion coefficient calculates to be 10- 13 cm2 s-1 when

the polymer matrix is a copolymer of vinyl chloride and vinyl acetate (VYHH from Union Carbide). Such data provide a useful model system that allows us to test theory based

on simple manipulations in the laboratory. This approach is

invaluable in the preparation of other coatings which display constant release rate characteristics.

Constant Release Rate

Coatings

Clearly one cannot expect constant long term release

from monolithic coatings such as those described above.

There are ways to achieve such fluxes, but each requires

manipulation of the coating in some fashion. For non-ablative coatings, the requirement is that the driving forces for

molecular diffusion and the resistances to diffusion are constant with time. Constant release rates can be achieved by

keeping the local concentration of agent at a steady value

throughout the lifetime of the coating and simultaneously

restricting the diffusional pathway to a constant length. This

effort gives rise to two concepts, both involving the creation

of a domain of very high concentration of agent that is surrounded by a layer of material to control flux of the AF agent.

The first of these ideas is the so called “reservoir membrane”.

A reservoir membrane is a two-layer coating where the base

layer is composed of pure or nearly pure AF agent. The base

layer has a top coat applied applied in the form of a polymer

which creates a membrane over the AF agent and acts to

control the diffusion of the agent to the surface. As long as

the agent remains near its initial concentration value, the

overall flux will be constant because the diffusional pathway is a constant length. Figure 8 shows a “Q vs time” plot

Figure 6

Quantity of Sea Nine 211 released as a function of time for a

monolithic coating initially containing Sea Nine 211 at 20-35

wt %.

Figure 7

Quantity of Sea Nine 211 released versus square root of time

for a monolithic coating initially containing Sea Nine 211 at

20-35 wt %.
56 Naval Research Reviews

for such a coating in which the base layer was composed of

a highly concentrated phase of Sea-Nine 211 dissolved and

dispersed in a plasticizer (SanticizerTM from Monsanto Co.).

The top layer was a room temperature vulcanized silicone

coating (RTV3140 from Dow Corning Co.). From these

data, the release rate, as judged by the slope of the line fit to

the data, is quite constant over the nearly 90 days of the test.

In this case the flux was 22 μg cm-2 d-1.

A second technique to achieve steady release rates is

microencapsulation of an AF agent prior to its addition to

the polymer matrix. Microencapsulation can be achieved

using a pure agent or a highly concentrated solution of agent

and a polymer capsule that is insoluble with the matrix polymer. In this case, the release rate will depend on control of

the slowed or controlled diffusion through the capsule wall

and much more rapid diffusion through the polymer matrix.

Another controlling parameter is the size distribution of the

microcapsules. In contrast to the concentration-dependent

release kinetics from the monolithic coatings containing SeaNine 211 in VYHH resin (Fig. 6), microencapsulation provides an effective means for achieving constant release rates

over moderate periods of time (Fig. 9). The AF agent was

dissolved in tricresyl phosphate at 45 weight % prior to encapsulation and the coating was prepared to contain 20 weight

% AF agent in the final coating. After the first few days, the

release of Sea-Nine 211 was quite steady at about 5 μg cm-2 d-1

for a 100 day period. When time scales expand well beyond

100 d, release rates will likely decline as the concentration

of the AF agent within the microcapsules diminishes. One

step to alleviate this decline somewhat is to encapsulate pure

AF agent.

Evaluation of Antifouling

Performance

As with any coating containing one or more AF agents,

it is critical to know the minimum release rate needed for

that agent to be effective in real world settings. Experience

teaches us that such rates may differ among the diversity of

marine organisms known to be troublesome foulers in coastal

regions worldwide. The diversity in minimum effective release rates must be related in part to the mode of action of a

particular AF agent, as described earlier for zosteric acid.

While steady release rates from coatings are a final goal,

when we evaluate long term effectiveness of a coating-AF

agent complex, the use of variable release rate coatings can

be extremely valuable in determining the minimum release

rate that is effective for problematic fouling species.

Figure 8

Quantity of Sea Nine 211 released versus time for a reservoir

membrane coating.

Figure 9

Release rate of Sea Nine 211 at 20 and 25 wt % loading

versus time for a microcapsule based coating.
Four/1997 57

Minimum Release Rate

Studies

We have taken advantage of the variable release rate

characteristics of monolithic coatings to establish the minimum flux of Sea Nine 211 necessary to control settlement

by a variety of fouling species by conducting field trials in

both Atlantic and Pacific Test sites. Here we prepared a variety of coatings based on VYHH polymer matrix with different loadings of Sea-Nine 211 dissolved in them. The test

materials were produced by coating cylindrical rods of glass

filled epoxy with the various coating formulations. The rods

were then placed in the marine environment and tested

weekly for the extent and nature of fouling over a 28 day

period following Rittschof et al.12. This length of time is

sufficient to determine the minimum effective flux necessary to minimize fouling of particular target organisms.

Simultaneously, we measured release rates in the laboratory

and have been able to establish substantial correlation between the release rate for Sea-Nine 211 in VYHH resin and

the extent of fouling for a diverse group of temperate and

tropical species19. Because the polymer matrix of the coating may have an effect upon the minimum release rate determination, we normalize the fouling data to a control sample

of the same resin without any AF agent. Figure 10 shows

the type of results that can be obtained from such a manipulation. These data represent results for arborescent hydrozoans obtained from field trials conducted in different oceans

over different seasons of the year. The various data points

represent samples analyzed at 7, 14, 21, and 28 days. A flux

above 5 to 7 μg cm-2 d-1 keeps the hydrozoan from fouling

the AF + VYHH coating. Similar data are available for diatoms, barnacles, amphipods and polychaete worms, including H. elegans and colonial tunicates.19 Interestingly, the

minimum effective release rates for all of these species excepting diatoms and barnacles were also in the

5 to 7 μg cm-2 d-1. Both diatoms and barnacles required

greater fluxes, estimated at this point to be in the 15 to

25 μg cm-2 d-1 range. The strength of this approach comes

from the ease with which effective concentrations can be

determined for a variety of troublesome fouling organisms

in their real world habitats but with an accelerated timeframe

for data acquisition. This approach is new but appears very

powerful in being able to diagnose minimum effective release rates for the study of any other AF agent and/or another matrix resin.

Constant Release Rate

Studies

Results from minimum release rate determinations can

be very helpful to formulate and rapidly test new coatings

which deliver the agent at constant rates. We extended these

studies by preparing Sea-Nine 211 microcapsule based coatings (with VYHH matrix polymer) to deliver Sea-Nine 211

at fluxes significantly above and just below the 5 to

Figure 10

Normalized extent of fouling of arborescent hydrozoans

versus release rate of Sea Nine 211 for a monolithic coating.

Figure 11

Normalized extent of fouling of arborescent hydrozoans

versus time for a microcapsule based coating at two different

release rates of Sea Nine 211.
58 Naval Research Reviews

7 μg cm-2 d-1 values determined as above. These samples

were tested in Pearl Harbor waters for a period of 60 days

and the results for arborescent hydrozoans are shown in Figure 11. Clearly at a flux of 11 to 12 μg cm-2 d-1, the fouling

of this species has been well controlled, while a flux of 4

to 5 μg cm-2 d-1 resulted in only partial control. As expected

from the minimum release rate study, these two microcapsule-based coatings performed poorly against diatoms. In

Figure 12, the test coatings demonstrate different partial levels of control. The standard ablative copper-based marine

paint (BRA 640) did not control diatom fouling any better

than the higher releasing (i.e. 11-12 μg cm -2 d-1) Sea-Nine

211 coating (Fig. 12). All of these results demonstrate the

utility of the minimum release rate procedure described above

as well as the practicality of microcapsule based coatings.

Further it demonstrates that copper ablative coatings with

the addition of an AF agent such as Sea-Nine 211 may not

offer significant AF advantages over formulations containing just copper.

Future Directions

The directions outlined here point out new arenas that

result from novel combinations of polymer chemistry, natural products chemistry with marine biology and ecology to

solve the age-old problems associated with biofouling. There

will likely always be a need to search for new AF agents that

will both be (i) broadly effective at low flux rates and (ii)

even more benign to the marine environment because the

settling organisms themselves may evolve to new fouling

strategies. In this kind of ecological arms race, we have

accelerated testing and other innovations to identify new

sources of non-toxic natural products from diverse classes

of marine organisms screened with larval settlement as a

rapid, powerful bioassay. Future environmental concerns

will also focus on safety associated with painting and drydock

operations. Future complex multicomponent coatings may

payoff in novel ways by minimizing fouling among complex biological communities world-wide. Finally, other

matrix polymers will have to be examined or even created

as we take advantage of materials with lower surface energy, easy release characteristics and other technological

breakthroughs brought about for the coatings of the next

century.

Acknowledgments

This work has been supported by the Office of Naval

Research under contract numbers N00014-91-J-1536 and

N00014-92-J-1516 to DCS, N00014-92-J-1687 and N00014-

95-I-G007 to RCZ, and N00014-91-J-1932 and N00014-

94-I-0196to CMS for which the authors are grateful. The

collaboration among academic, US Navy and industrial scientists and engineers involved in this program has been critical to the advancements described in this article.

Biographies

Dr. Donald Sundberg is the Vice President of Research

and Public Service at the University of New Hampshire. He

is also the Director of the Polymer Research Group within

the Department of Chemical Engineering. His research interests are in controlled release technology, surface science,

and in polymer reaction kinetics and composite particle morphology of emulsion polymers.

Dr. Niraj Vasishtha is the Manager of the Microencapsulation and Process Research Section in the Chemistry and

Chemical Engineering Division at Southwest Research Institute. His primary research focus is in the microencapsulation of food, pharmaceutical, agricultural, and consumer

products. His other interests are in controlled release technology, polymer coatings and blends.

Dr. Richard C. Zimmerman is Associate Research Scientist at Moss Landing Marine Laboratories, San Jose State

University. His research interests are in the ecological physiology of marine autotrophs, particularly with regard to environmental influences on photosynthesis, nutrient uptake,

chemical composition, resource partitioning and growth of

seagrasses and seaweeds.

Dr. Celia M. Smith is Associate Professor of Botany,

Botany Department of the University of Hawaii and active

in the field of biofouling research with emphases on algal

Figure 12

Normalized extent of fouling of diatoms versus time for a

microcapsule based coating at two different release rates of

Sea Nine 211. Extent of fouling for copper oxide control

coating (BRA 640) included for comparison.
Four/1997 59

recognition of potential fouling surfaces via settlement cues

and identification of new non-toxic antifouling agents from

algae.

REFERENCES

1. Brady, S.A., Johns, R.B., & Smith, J.D. 1994. Mar.

Poll. Bull. 28:230-4.

2. Crank, J. 1975. The Mathematics of Diffusion.

Oxford Clarendon Press. 2nd ed., pp 77-9.

3. Davis, A.R., Targett, N.M., McConnell, O.J. &

Young, C.M. 1989. Bioorg. Mar. Chem. 3:85-114.

4. Filion-Myklebust, C. & Norton, T.A. 1981. Mar.

Biol. Let. 2:45-51.

5. Hadfield, M. G., Unabia, C. C., Smith, C. M.,

Michael, T. M. 1994. Settlement preferences of the

ubiquitous fouler Hydroides elegans. In: Thompson,

M. F., Nagabhushanam, R., Sarojini, R., Fingerman,

M. (eds.) Recent developments in biofouling control.

Oxford and IBH Publishing Co. New Delhi, pp 67-

74.

6. Hay, M. E. & Fenical, W. 1988. Ann. Rev. Ecol.

Syst. 19:111-145.

7. Higuchi, T. 1963. J. Pharm. Sci. 52: 1145-9

8. Michael, T. & Smith, C. M. 1995. Mar. Ecol. Prog.

Ser. 119:229-236.

9. Patton, W.K. 1972. Bull. Mar. Sci. 22:419-31.

10. Pearce, F. 1995. New Scientist 45:7.

11. Rittschof, D., Hooper, I.R., Branscomb, E.S., &

Costlow, J. D. 1985. J. Chem. Ecol. 11:551-64.

12. Rittschof D., Clare, A. S., Gerhart, D. J.,

Boneventura, J., Smith C. M., & Hadfield, M.G.

1992. Biofouling 6: 181-92.

13. Rublee, P.A., Lasker, H.R., Gottfried M. & Roman,

M.R. 1980. Bull. Mar. Sci. 30:888-93.

14. Sieburth, J.M. & Conover J.T. 1965. Nature 208:52-

3.

15. Stephenson, M.D. & Leonard, G.H. 1994. Mar.

Poll. Bull. 28:148-53.

16. Targett, N.M. 1988. Allelochemistry in marine

organisms: Chemical fouling and antifouling

strategies. In: Thompson, M. F., Sarojini, R.,

Nagabhushanam, R. (eds) Marine Biodeterioration.

Oxford and IBH Publ. Co. Pvt. Ltd., New Delhi,

pp. 609-18.

17. Targett, N. M., Bishop, S. S., McConnell, 0. J. &

Yoder, J. A. 1983. J. Chem. Ecol. 9:817-29.

18. Todd, J.S., Zimmerman, R.C., Crews P. & Alberte

R.S. 1993. Phytochem. 34:401-4.

19. Vasishtha N., Sundberg D., & Rittschof D. 1995.

Biofouling 9:1-16.

20. Vrolijk, N.H., Targett, N.M., Baier, R. E. & Meyer A.

E. 1990. Biofouling 2:39-54.

21. Walters, L.J., Hadfield M.G., & Smith, C.M. 1996.

Marine Biology 126:383-93.

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1989. Agric. Biol. Chem. 53:2827-2830.
60 Naval Research Reviews

Introduction

Laboratory and field trials have demonstrated that

methylsilicone-based coatings having critical surface tensions

between 20 and 25 mN/m allow the easiest mechanical detachment of biofouling from coated test panels.1

 Continuing evaluations of such coated panels and trash racks at test

sites in Western New York State have confirmed and extended

these findings more than 5 years in zebra mussel-infested

fresh waters.2

 Coatings which, in addition, contain elutable

oils have displayed an apparent further resistance to initial

colonization by barnacles and zebra mussels, but this early

benefit does not necessarily carry over to the brush-removal

forces required for cleaning of the once-fouled coatings.

Some of the elastomeric methysilicone coatings are prone to

cutting and abrasion damage, limiting their suitability for

heavy-duty use and/or situations requiring periodic cleaning. Since standard tests for abrasion and wear (e.g. falling

sand test) developed for paints are not applicable to elastomeric coatings, we developed a brush abrasion test to sequentially evaluate fouling-release coatings after increasing

numbers of wet brushing cycles. Both laboratory and fielddeployed devices are in routine use for this purpose.

The Test Protocol

Table 1 lists the series of analyses included in our protocol for evaluation of fouling-release coatings.

Comprehensive contact angle analyses (Figure 1) are

used to determine the critical surface tensions and the composite surface free energies of all samples, as well as the

“polar” (including acid and base descriptors) and “disperCertification of

Properties of

Nontoxic FoulingRelease Coatings

Exposed to Abrasion

and Long-Term

Immersion

Robert E. Baier, Anne E. Meyer, and Robert L. Forsberg, Industry/University Center for Biosurfaces, State University of New York at Buffalo, Buffalo, NY
Four/1997 61

sion” components of the surface energies.5,6 Contact angle

measurements are sensitive to the outermost atomic layers

of the sample surfaces, so this technique is most pertinent to

characterization of coatings both before immersion (pre-exposure) and after removal of replicates from test sites at various evaluation intervals.

Multiple-attenuated-internal-reflection infrared

(MAIR-IR) spectroscopy is applied for the evaluation of the

chemical composition of the outermost, approximately, 1000

Angstroms of each coating’s surface zone. Methyl, methylene, silicone, epoxy, carbonyl, amide, and other covalently

bound moieties are easily detected by this method (Figure 2).

Coating or environmental compounds transferred from the

samples to internal reflection test plates (e.g. elutable oils,

diatomaceous slimes) are easily detected. MAIR-IR spectroscopy, used for characterization of both pre- and post-exposure coatings, generally has revealed biofilm buildup on

all the latter specimens.

The rotating brush abrasion and wear test was developed in our laboratory to overcome shortcomings of other

accepted paint film test methods.8

 Falling sand (or other

abrasive material) tests do not provide meaningful data for

the easy-release, elastomeric silicone-based coatings; prior

test procedures that utilize brushes evaluate only the outermost aspects of coating surfaces, indicating whether the surface finish dulls within a specific test interval (the brush is

unloaded in these procedures).

The rotating brush abrasion and wear test discussed

here is customized for foul-release coatings. Standardized

nylon bristle brushes are fixed to a rotating wheel, which is

positioned against a surface bearing the test coating (Figure 3). The contact pressure (normal to the direction of the

brush stroke), is controlled to be approximately 40 psi. This

brush load was selected on the basis of numerous laboratory

trials to determine the minimum forces required to remove

all visible biofouling (including slime films) from coatings

that showed consistent easy-release properties in the field.9

Thus, our tests provide realistic predictions for coating wear

Figure 1

Comprehensive contact angle analysis, as originally described by Young3, is an affordable, sensitive, and reliable method of

monitoring the adhesive properties of any coating. Contact angle measurements characterize the outermost 4-5 Angstroms

of a sample. The “critical surface tension” is derived from a graph [Zisman plot], which relates the surface tensions of a

variety of diagnostic liquids to the cosines of the angles the liquid droplets make with a given coating surface.4 Water contact

angles, alone, are insufficient, and often misleading, with regard to fouling performance over the long term.

Table 1

Coating Evaluation Methods.
62 Naval Research Reviews

from sequential brush-based cleaning events. In our laboratory device, the brushes move through the top of a reservoir

of either freshwater or seawater (depending on the requirements of the study), carrying up clean water before moving

across the surface of the coated coupon at a rate of approximately 200 brush strokes per minute. Removed coating fragments collect at the bottom of the water reservoir for later

analysis. The coupons, water, debris, and brushes, all, can

be saved for a complete evaluation of the wear process. In

the field-deployed device (Figure 4), the brushes trace a circular path that leaves behind an annular “ring” pattern in the

fouling layer.10 Pre-exposure brushing of coated panels results in easily recognized “wear rings” on the coating surfaces, to allow later inspection of differences in fouling

attachment to the abraded v. as-coated regions of the panels.

In the lab test, the coatings are characterized by comprehensive contact angle analyses (to determine sample critical surface tension, which indicates retention or loss of fouling-release character), MAIR-IR spectroscopy (for the

chemical signature of exudates and pressure-transferred residues), and depth of wear measurements after each of four

intervals of rotating brush abrasion and wear: 20,000 brush

strokes, 50,000 strokes, 90,000 strokes, and 145,000 strokes.

Exemplary results reported here are for coatings prepared

either by the manufacturer or by experienced applicators, in

the identical manner employed for coatings exposed in the

field.

The oil uptake/swelling test is in the preliminary stages

of development in our laboratory. Due to the relatively open,

elastomeric structure of most fouling-release coatings, it is

possible that dimensional changes due to oil-uptake (recall

the fundamental “rule of thumb” in chemistry: “like likes

like”) or exudation will affect the long-term stabilities of the

coatings. Coated coupons are immersed at the waterline (i.e.

partial immersion) in a deep container of water into which

#2 fuel oil is vortexed in sufficient quantity to saturate the

water and allow a thin (iridescent) oil film to remain on its

surface. This simulates harbor situations, where coated ships

might be on station for extended periods between operational

assignments. Some of the fuel remains suspended in the

water column. Observations are made for coating dimension changes and any delamination from base coats or the

original substratum.

Figure 2

Internal reflection infrared (IR) spectroscopy7 is a simple and

powerful technique for learning the chemical composition of

coatings and their transferrable residues. For thick-layer

materials, the sampling depth of the technique ranges from

approximately 1 micrometer (for nonabsorbing materials), to

as little as 1000 Angstroms for absorbers. Superficial

residues, as little as 10-15 Angstroms in thickness, are easily

detected by re-analysis of the internal reflection test plate

after removal of the primary coating sample. For foulingrelease coatings, the amounts of silicone (or other materials)

transferred to the test surface via pressure-contact is an

excellent indicator of the presence of elutable or releasable

components in the sample.

Figure 3

Schematic of the rotating brush abrasion unit, used to evaluate

a coating’s resistance to cleaning-related damage.

Determination of the worn coating’s new operational critical

surface tension is key to determining whether the coating will

maintain its fouling-release character after being abraded.
Four/1997 63

Coatings Wear

Differently

The following bar charts illustrate the types of differences found during rotating brush abrasion and wear tests

performed with pre-exposure coatings. Figures 5-7 summarize the average depths of wear and post-abrasion results from

comprehensive contact angle analyses after each test interval. Table 2 gives a general description of each of the 16

coatings represented in Figures 5-7.

Similar Wear Depths

Reveal Different

Chemistries

Brush abrasion and wear does remove the fouling-release layers of some coatings, resulting in loss of performance in the field. Among the coatings tested in this group,

oil-free and low-oil-content coatings tended to be more abrasion-resistant than oil-laden coatings in the test procedures

reported here. Depth of wear does not simply correlate directly with loss of fouling-release properties, due to the variations of topcoat thicknesses and tiecoat compositions from

coating to coating. This is an important observation, since it

shows that measurements of abrasion resistance, alone, are

not sufficient to judge retention of fouling-release qualities.

Continuing field inspections demonstrate that coatings

having initial critical surface tensions between 20 and 25

mN/m have better fouling-release properties than coatings

with critical surface tensions outside of this range.1,11 Within

the optimal range, however, some coatings perform consistently better than others.8

 Current studies focus on the effects of surface “polarity” and surface “compliance” on the

organization of vicinal water that may influence biological

adhesion strengths and drag phenomena.12,13

Some fouling-release coatings tend to release oils, and

others to imbibe floating and suspended oils. Whether these

events significantly affect field performance is not yet known.

Retention of easy-release properties has been observed for

many such coatings over 5-year observation periods, and

most are still performing well at this writing. Previously

reported observations note that some coatings actually seem

Figure 4

Schematic of the underwater unit used in field brush shear tests to evaluate ease of fouling removal. Brush loading is similar

to that in the lab unit (see Figure 3). A simple scoring system is used to evaluate the extent of fouling removed in the test. An

“excellent” (Type 1pattern) score indicates that all visible fouling was removed from both the brush path and from the central

area, which is untouched by the brushes and is affected only by turbulent water action.
64 Naval Research Reviews

to improve in fouling-release character with exposure in the

field. Others that do degrade generally fail a “rub test”, in

which coatings exposed for more than 2 years in the field

reveal easy delamination (topcoat from tiecoat). Delamination from steel or concrete substrata has been rare. The occurrence of release-layer delamination is not surprising, given

the difficulty of binding such low-energy materials to anticorrosion undercoats. Manufacturers have continued to work

on and improve this aspect of their coating systems, since it

is now clear that a balanced evaluation of fouling-release

coatings must include abrasion resistance (with maintenance

of surface chemical integrity), resistance to oil-uptake and

swelling, and resistance to delamination as a function of exposure time, as well as minimization of the relative strengths

of fouling adhesion.

Acknowledgments

Different aspects of the work reported here have been

sponsored by ONR Grant #N00014-89-J-3101, the Environmental Security Technology Certification Program, Niagara

Mohawk Power Corporation (Syracuse, NY), and Parsons

Power (Reading, PA). The authors thank Lawler, Matusky

& Skelly Engineers (Pearl River, NY) for their contributions

to the field studies.

Table 2

Description of Coatings Evaluated by Rotating Brush Abrasion

and Wear Test.

Figure 5

Results of rotating brush abrasion and wear test for Coatings

A-E [see Table 2]. Stars indicate that the average depth of

wear was less than 0.3 thousandths of an inch. Solid circles

indicate that the abraded coating no longer has a critical

surface tension in the “fouling-release zone”. Coating A was

not designed as a fouling-release material.

Figure 6

Results of rotating brush abrasion and wear test for Coatings

F-J [see Table 2]. Symbols are defined in Figure 1 caption.

Figure 7

Results of rotating brush abrasion and wear test for Coatings

K-P [see Table 2]. Symbols are defined in Figure 1 caption.

Note that the critical surface tension of Coating O was not in

the “fouling-release zone”, even before the first cycle of

abrasion testing.
Four/1997 65

Biographies

The authors are with the National Science Foundationsponsored Industry/University Cooperative Research Center for Biosurfaces (IUCB), of which ONR was a founding

member. Dr. Robert Baier trained in Engineering Physics

(B.E.S., Cleveland State University, 1962), Biophysics

(Ph.D., State University of New York at Buffalo, 1966), and

Surface Chemistry (NRC/NAS Postdoctoral at NRL, 1966-

68) before joining Cornell Aeronautical Laboratory (1968-

84). He joined the faculty of State University of New York

at Buffalo on a full-time basis in 1984. Dr. Baier is the Executive Director of IUCB, which involves research activities at 4 different universities in the U.S. Dr. Anne Meyer,

Director and Principal Research Scientist at the Center’s

University at Buffalo site, received her B.S. in Chemistry

from SUNY College at Potsdam in 1973 and her doctorate

in prosthetic materials from Lund University, Sweden, in

1990. She joined the technical staff of SUNY/Buffalo in

1986, after twelve years with Calspan Corporation (formerly

Cornell Aeronautical Laboratory). She was Principal Chemist

and Head of Calspan’s Surface Science Section during 1983-

1986. Mr. Robert Forsberg was originally trained in avionics in the U.S.Marine Corps, completed a B.S. in Biology at

State University of New York at Buffalo in 1986, and worked

in the materials fabrication industry before joining IUCB as

a Research Support Specialist in 1987.

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Initial fouling of nontoxic coatings in fresh, brackish,

and sea water. Can. J. Chem. Eng. 66:55-62.

2. Evaluation of nontoxic fouling-release coatings as a

means of reducing the attachment of zebra mussels

(Dreissena polymorpha) at Great Lakes generating

stations. 1997. Final project report prepared by

Lawler, Matusky & Skelly Engineers LLP and the

Industry/University Center for Biosurfaces, SUNY/

Buffalo for Niagara Mohawk Power Corporation,

Syracuse, NY.

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7. Harrick, N.J. 1960. Surface chemistry from spectral

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Proceedings, 4th International Zebra Mussel Conference. Electric Power Research Institute Report #TR104029, pp 273-290.

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